perlre
(1)
Name
perlre - Perl regular expressions
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLRE(1)
NAME
perlre - Perl regular expressions
DESCRIPTION
This page describes the syntax of regular expressions in
Perl.
If you haven't used regular expressions before, a quick-
start introduction is available in perlrequick, and a longer
tutorial introduction is available in perlretut.
For reference on how regular expressions are used in
matching operations, plus various examples of the same, see
discussions of "m//", "s///", "qr//" and "??" in "Regexp
Quote-Like Operators" in perlop.
Modifiers
Matching operations can have various modifiers. Modifiers
that relate to the interpretation of the regular expression
inside are listed below. Modifiers that alter the way a
regular expression is used by Perl are detailed in "Regexp
Quote-Like Operators" in perlop and "Gory details of parsing
quoted constructs" in perlop.
m Treat string as multiple lines. That is, change "^" and
"$" from matching the start or end of the string to
matching the start or end of any line anywhere within
the string.
s Treat string as single line. That is, change "." to
match any character whatsoever, even a newline, which
normally it would not match.
Used together, as "/ms", they let the "." match any
character whatsoever, while still allowing "^" and "$"
to match, respectively, just after and just before
newlines within the string.
i Do case-insensitive pattern matching.
If "use locale" is in effect, the case map is taken from
the current locale. See perllocale.
x Extend your pattern's legibility by permitting
whitespace and comments.
p Preserve the string matched such that ${^PREMATCH},
${^MATCH}, and ${^POSTMATCH} are available for use after
matching.
g and c
Global matching, and keep the Current position after
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failed matching. Unlike i, m, s and x, these two flags
affect the way the regex is used rather than the regex
itself. See "Using regular expressions in Perl" in
perlretut for further explanation of the g and c
modifiers.
These are usually written as "the "/x" modifier", even
though the delimiter in question might not really be a
slash. Any of these modifiers may also be embedded within
the regular expression itself using the "(?...)" construct.
See below.
The "/x" modifier itself needs a little more explanation.
It tells the regular expression parser to ignore most
whitespace that is neither backslashed nor within a
character class. You can use this to break up your regular
expression into (slightly) more readable parts. The "#"
character is also treated as a metacharacter introducing a
comment, just as in ordinary Perl code. This also means
that if you want real whitespace or "#" characters in the
pattern (outside a character class, where they are
unaffected by "/x"), then you'll either have to escape them
(using backslashes or "\Q...\E") or encode them using octal,
hex, or "\N{}" escapes. Taken together, these features go a
long way towards making Perl's regular expressions more
readable. Note that you have to be careful not to include
the pattern delimiter in the comment--perl has no way of
knowing you did not intend to close the pattern early. See
the C-comment deletion code in perlop. Also note that
anything inside a "\Q...\E" stays unaffected by "/x". And
note that "/x" doesn't affect whether space interpretation
within a single multi-character construct. For example in
"\x{...}", regardless of the "/x" modifier, there can be no
spaces. Same for a quantifier such as "{3}" or "{5,}".
Similarly, "(?:...)" can't have a space between the "?" and
":", but can between the "(" and "?". Within any delimiters
for such a construct, allowed spaces are not affected by
"/x", and depend on the construct. For example, "\x{...}"
can't have spaces because hexadecimal numbers don't have
spaces in them. But, Unicode properties can have spaces, so
in "\p{...}" there can be spaces that follow the Unicode
rules, for which see "Properties accessible through \p{} and
\P{}" in perluniprops.
Regular Expressions
Metacharacters
The patterns used in Perl pattern matching evolved from
those supplied in the Version 8 regex routines. (The
routines are derived (distantly) from Henry Spencer's freely
redistributable reimplementation of the V8 routines.) See
"Version 8 Regular Expressions" for details.
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In particular the following metacharacters have their
standard egrep-ish meanings:
\ Quote the next metacharacter
^ Match the beginning of the line
. Match any character (except newline)
$ Match the end of the line (or before newline at the end)
| Alternation
() Grouping
[] Bracketed Character class
By default, the "^" character is guaranteed to match only
the beginning of the string, the "$" character only the end
(or before the newline at the end), and Perl does certain
optimizations with the assumption that the string contains
only one line. Embedded newlines will not be matched by "^"
or "$". You may, however, wish to treat a string as a
multi-line buffer, such that the "^" will match after any
newline within the string (except if the newline is the last
character in the string), and "$" will match before any
newline. At the cost of a little more overhead, you can do
this by using the /m modifier on the pattern match operator.
(Older programs did this by setting $*, but this practice
has been removed in perl 5.9.)
To simplify multi-line substitutions, the "." character
never matches a newline unless you use the "/s" modifier,
which in effect tells Perl to pretend the string is a single
line--even if it isn't.
Quantifiers
The following standard quantifiers are recognized:
* Match 0 or more times
+ Match 1 or more times
? Match 1 or 0 times
{n} Match exactly n times
{n,} Match at least n times
{n,m} Match at least n but not more than m times
(If a curly bracket occurs in any other context, it is
treated as a regular character. In particular, the lower
bound is not optional.) The "*" quantifier is equivalent to
"{0,}", the "+" quantifier to "{1,}", and the "?" quantifier
to "{0,1}". n and m are limited to non-negative integral
values less than a preset limit defined when perl is built.
This is usually 32766 on the most common platforms. The
actual limit can be seen in the error message generated by
code such as this:
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$_ **= $_ , / {$_} / for 2 .. 42;
By default, a quantified subpattern is "greedy", that is, it
will match as many times as possible (given a particular
starting location) while still allowing the rest of the
pattern to match. If you want it to match the minimum
number of times possible, follow the quantifier with a "?".
Note that the meanings don't change, just the "greediness":
*? Match 0 or more times, not greedily
+? Match 1 or more times, not greedily
?? Match 0 or 1 time, not greedily
{n}? Match exactly n times, not greedily
{n,}? Match at least n times, not greedily
{n,m}? Match at least n but not more than m times, not greedily
By default, when a quantified subpattern does not allow the
rest of the overall pattern to match, Perl will backtrack.
However, this behaviour is sometimes undesirable. Thus Perl
provides the "possessive" quantifier form as well.
*+ Match 0 or more times and give nothing back
++ Match 1 or more times and give nothing back
?+ Match 0 or 1 time and give nothing back
{n}+ Match exactly n times and give nothing back (redundant)
{n,}+ Match at least n times and give nothing back
{n,m}+ Match at least n but not more than m times and give nothing back
For instance,
'aaaa' =~ /a++a/
will never match, as the "a++" will gobble up all the "a"'s
in the string and won't leave any for the remaining part of
the pattern. This feature can be extremely useful to give
perl hints about where it shouldn't backtrack. For instance,
the typical "match a double-quoted string" problem can be
most efficiently performed when written as:
/"(?:[^"\\]++|\\.)*+"/
as we know that if the final quote does not match,
backtracking will not help. See the independent
subexpression "(?>...)" for more details; possessive
quantifiers are just syntactic sugar for that construct. For
instance the above example could also be written as follows:
/"(?>(?:(?>[^"\\]+)|\\.)*)"/
Escape sequences
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Because patterns are processed as double quoted strings, the
following also work:
\t tab (HT, TAB)
\n newline (LF, NL)
\r return (CR)
\f form feed (FF)
\a alarm (bell) (BEL)
\e escape (think troff) (ESC)
\033 octal char (example: ESC)
\x1B hex char (example: ESC)
\x{263a} long hex char (example: Unicode SMILEY)
\cK control char (example: VT)
\N{name} named Unicode character
\N{U+263D} Unicode character (example: FIRST QUARTER MOON)
\l lowercase next char (think vi)
\u uppercase next char (think vi)
\L lowercase till \E (think vi)
\U uppercase till \E (think vi)
\Q quote (disable) pattern metacharacters till \E
\E end either case modification or quoted section (think vi)
Details are in "Quote and Quote-like Operators" in perlop.
Character Classes and other Special Escapes
In addition, Perl defines the following:
Sequence Note Description
[...] [1] Match a character according to the rules of the bracketed
character class defined by the "...". Example: [a-z]
matches "a" or "b" or "c" ... or "z"
[[:...:]] [2] Match a character according to the rules of the POSIX
character class "..." within the outer bracketed character
class. Example: [[:upper:]] matches any uppercase
character.
\w [3] Match a "word" character (alphanumeric plus "_")
\W [3] Match a non-"word" character
\s [3] Match a whitespace character
\S [3] Match a non-whitespace character
\d [3] Match a decimal digit character
\D [3] Match a non-digit character
\pP [3] Match P, named property. Use \p{Prop} for longer names.
\PP [3] Match non-P
\X [4] Match Unicode "eXtended grapheme cluster"
\C Match a single C-language char (octet) even if that is part
of a larger UTF-8 character. Thus it breaks up characters
into their UTF-8 bytes, so you may end up with malformed
pieces of UTF-8. Unsupported in lookbehind.
\1 [5] Backreference to a specific capture buffer or group.
'1' may actually be any positive integer.
\g1 [5] Backreference to a specific or previous group,
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\g{-1} [5] The number may be negative indicating a relative previous
buffer and may optionally be wrapped in curly brackets for
safer parsing.
\g{name} [5] Named backreference
\k<name> [5] Named backreference
\K [6] Keep the stuff left of the \K, don't include it in $&
\N [7] Any character but \n (experimental). Not affected by /s
modifier
\v [3] Vertical whitespace
\V [3] Not vertical whitespace
\h [3] Horizontal whitespace
\H [3] Not horizontal whitespace
\R [4] Linebreak
[1] See "Bracketed Character Classes" in perlrecharclass for
details.
[2] See "POSIX Character Classes" in perlrecharclass for
details.
[3] See "Backslash sequences" in perlrecharclass for
details.
[4] See "Misc" in perlrebackslash for details.
[5] See "Capture buffers" below for details.
[6] See "Extended Patterns" below for details.
[7] Note that "\N" has two meanings. When of the form
"\N{NAME}", it matches the character whose name is
"NAME"; and similarly when of the form "\N{U+wide hex
char}", it matches the character whose Unicode ordinal
is wide hex char. Otherwise it matches any character
but "\n".
Assertions
Perl defines the following zero-width assertions:
\b Match a word boundary
\B Match except at a word boundary
\A Match only at beginning of string
\Z Match only at end of string, or before newline at the end
\z Match only at end of string
\G Match only at pos() (e.g. at the end-of-match position
of prior m//g)
A word boundary ("\b") is a spot between two characters that
has a "\w" on one side of it and a "\W" on the other side of
it (in either order), counting the imaginary characters off
the beginning and end of the string as matching a "\W".
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(Within character classes "\b" represents backspace rather
than a word boundary, just as it normally does in any
double-quoted string.) The "\A" and "\Z" are just like "^"
and "$", except that they won't match multiple times when
the "/m" modifier is used, while "^" and "$" will match at
every internal line boundary. To match the actual end of
the string and not ignore an optional trailing newline, use
"\z".
The "\G" assertion can be used to chain global matches
(using "m//g"), as described in "Regexp Quote-Like
Operators" in perlop. It is also useful when writing
"lex"-like scanners, when you have several patterns that you
want to match against consequent substrings of your string,
see the previous reference. The actual location where "\G"
will match can also be influenced by using "pos()" as an
lvalue: see "pos" in perlfunc. Note that the rule for zero-
length matches is modified somewhat, in that contents to the
left of "\G" is not counted when determining the length of
the match. Thus the following will not match forever:
$str = 'ABC';
pos($str) = 1;
while (/.\G/g) {
print $&;
}
It will print 'A' and then terminate, as it considers the
match to be zero-width, and thus will not match at the same
position twice in a row.
It is worth noting that "\G" improperly used can result in
an infinite loop. Take care when using patterns that include
"\G" in an alternation.
Capture buffers
The bracketing construct "( ... )" creates capture buffers.
To refer to the current contents of a buffer later on,
within the same pattern, use \1 for the first, \2 for the
second, and so on. Outside the match use "$" instead of
"\". (The \<digit> notation works in certain circumstances
outside the match. See "Warning on \1 Instead of $1" below
for details.) Referring back to another part of the match
is called a backreference.
There is no limit to the number of captured substrings that
you may use. However Perl also uses \10, \11, etc. as
aliases for \010, \011, etc. (Recall that 0 means octal, so
\011 is the character at number 9 in your coded character
set; which would be the 10th character, a horizontal tab
under ASCII.) Perl resolves this ambiguity by interpreting
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\10 as a backreference only if at least 10 left parentheses
have opened before it. Likewise \11 is a backreference only
if at least 11 left parentheses have opened before it. And
so on. \1 through \9 are always interpreted as
backreferences. If the bracketing group did not match, the
associated backreference won't match either. (This can
happen if the bracketing group is optional, or in a
different branch of an alternation.)
In order to provide a safer and easier way to construct
patterns using backreferences, Perl provides the "\g{N}"
notation (starting with perl 5.10.0). The curly brackets are
optional, however omitting them is less safe as the meaning
of the pattern can be changed by text (such as digits)
following it. When N is a positive integer the "\g{N}"
notation is exactly equivalent to using normal
backreferences. When N is a negative integer then it is a
relative backreference referring to the previous N'th
capturing group. When the bracket form is used and N is not
an integer, it is treated as a reference to a named buffer.
Thus "\g{-1}" refers to the last buffer, "\g{-2}" refers to
the buffer before that. For example:
/
(Y) # buffer 1
( # buffer 2
(X) # buffer 3
\g{-1} # backref to buffer 3
\g{-3} # backref to buffer 1
)
/x
and would match the same as "/(Y) ( (X) \3 \1 )/x".
Additionally, as of Perl 5.10.0 you may use named capture
buffers and named backreferences. The notation is
"(?<name>...)" to declare and "\k<name>" to reference. You
may also use apostrophes instead of angle brackets to
delimit the name; and you may use the bracketed "\g{name}"
backreference syntax. It's possible to refer to a named
capture buffer by absolute and relative number as well.
Outside the pattern, a named capture buffer is available via
the "%+" hash. When different buffers within the same
pattern have the same name, $+{name} and "\k<name>" refer to
the leftmost defined group. (Thus it's possible to do things
with named capture buffers that would otherwise require
"(??{})" code to accomplish.)
Examples:
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s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
/(.)\1/ # find first doubled char
and print "'$1' is the first doubled character\n";
/(?<char>.)\k<char>/ # ... a different way
and print "'$+{char}' is the first doubled character\n";
/(?'char'.)\1/ # ... mix and match
and print "'$1' is the first doubled character\n";
if (/Time: (..):(..):(..)/) { # parse out values
$hours = $1;
$minutes = $2;
$seconds = $3;
}
Several special variables also refer back to portions of the
previous match. $+ returns whatever the last bracket match
matched. $& returns the entire matched string. (At one
point $0 did also, but now it returns the name of the
program.) "$`" returns everything before the matched
string. "$'" returns everything after the matched string.
And $^N contains whatever was matched by the most-recently
closed group (submatch). $^N can be used in extended
patterns (see below), for example to assign a submatch to a
variable.
The numbered match variables ($1, $2, $3, etc.) and the
related punctuation set ($+, $&, "$`", "$'", and $^N) are
all dynamically scoped until the end of the enclosing block
or until the next successful match, whichever comes first.
(See "Compound Statements" in perlsyn.)
NOTE: Failed matches in Perl do not reset the match
variables, which makes it easier to write code that tests
for a series of more specific cases and remembers the best
match.
WARNING: Once Perl sees that you need one of $&, "$`", or
"$'" anywhere in the program, it has to provide them for
every pattern match. This may substantially slow your
program. Perl uses the same mechanism to produce $1, $2,
etc, so you also pay a price for each pattern that contains
capturing parentheses. (To avoid this cost while retaining
the grouping behaviour, use the extended regular expression
"(?: ... )" instead.) But if you never use $&, "$`" or
"$'", then patterns without capturing parentheses will not
be penalized. So avoid $&, "$'", and "$`" if you can, but
if you can't (and some algorithms really appreciate them),
once you've used them once, use them at will, because you've
already paid the price. As of 5.005, $& is not so costly as
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the other two.
As a workaround for this problem, Perl 5.10.0 introduces
"${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}", which are
equivalent to "$`", $& and "$'", except that they are only
guaranteed to be defined after a successful match that was
executed with the "/p" (preserve) modifier. The use of
these variables incurs no global performance penalty, unlike
their punctuation char equivalents, however at the trade-off
that you have to tell perl when you want to use them.
Quoting metacharacters
Backslashed metacharacters in Perl are alphanumeric, such as
"\b", "\w", "\n". Unlike some other regular expression
languages, there are no backslashed symbols that aren't
alphanumeric. So anything that looks like \\, \(, \), \<,
\>, \{, or \} is always interpreted as a literal character,
not a metacharacter. This was once used in a common idiom
to disable or quote the special meanings of regular
expression metacharacters in a string that you want to use
for a pattern. Simply quote all non-"word" characters:
$pattern =~ s/(\W)/\\$1/g;
(If "use locale" is set, then this depends on the current
locale.) Today it is more common to use the quotemeta()
function or the "\Q" metaquoting escape sequence to disable
all metacharacters' special meanings like this:
/$unquoted\Q$quoted\E$unquoted/
Beware that if you put literal backslashes (those not inside
interpolated variables) between "\Q" and "\E", double-
quotish backslash interpolation may lead to confusing
results. If you need to use literal backslashes within
"\Q...\E", consult "Gory details of parsing quoted
constructs" in perlop.
Extended Patterns
Perl also defines a consistent extension syntax for features
not found in standard tools like awk and lex. The syntax is
a pair of parentheses with a question mark as the first
thing within the parentheses. The character after the
question mark indicates the extension.
The stability of these extensions varies widely. Some have
been part of the core language for many years. Others are
experimental and may change without warning or be completely
removed. Check the documentation on an individual feature
to verify its current status.
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A question mark was chosen for this and for the minimal-
matching construct because 1) question marks are rare in
older regular expressions, and 2) whenever you see one, you
should stop and "question" exactly what is going on. That's
psychology...
"(?#text)"
A comment. The text is ignored. If the "/x"
modifier enables whitespace formatting, a simple
"#" will suffice. Note that Perl closes the
comment as soon as it sees a ")", so there is no
way to put a literal ")" in the comment.
"(?pimsx-imsx)"
One or more embedded pattern-match modifiers, to
be turned on (or turned off, if preceded by "-")
for the remainder of the pattern or the remainder
of the enclosing pattern group (if any). This is
particularly useful for dynamic patterns, such as
those read in from a configuration file, taken
from an argument, or specified in a table
somewhere. Consider the case where some patterns
want to be case sensitive and some do not: The
case insensitive ones merely need to include
"(?i)" at the front of the pattern. For example:
$pattern = "foobar";
if ( /$pattern/i ) { }
# more flexible:
$pattern = "(?i)foobar";
if ( /$pattern/ ) { }
These modifiers are restored at the end of the
enclosing group. For example,
( (?i) blah ) \s+ \1
will match "blah" in any case, some spaces, and an
exact (including the case!) repetition of the
previous word, assuming the "/x" modifier, and no
"/i" modifier outside this group.
These modifiers do not carry over into named
subpatterns called in the enclosing group. In
other words, a pattern such as "((?i)(&NAME))"
does not change the case-sensitivity of the "NAME"
pattern.
Note that the "p" modifier is special in that it
can only be enabled, not disabled, and that its
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presence anywhere in a pattern has a global
effect. Thus "(?-p)" and "(?-p:...)" are
meaningless and will warn when executed under "use
warnings".
"(?:pattern)"
"(?imsx-imsx:pattern)"
This is for clustering, not capturing; it groups
subexpressions like "()", but doesn't make
backreferences as "()" does. So
@fields = split(/\b(?:a|b|c)\b/)
is like
@fields = split(/\b(a|b|c)\b/)
but doesn't spit out extra fields. It's also
cheaper not to capture characters if you don't
need to.
Any letters between "?" and ":" act as flags
modifiers as with "(?imsx-imsx)". For example,
/(?s-i:more.*than).*million/i
is equivalent to the more verbose
/(?:(?s-i)more.*than).*million/i
"(?|pattern)"
This is the "branch reset" pattern, which has the
special property that the capture buffers are
numbered from the same starting point in each
alternation branch. It is available starting from
perl 5.10.0.
Capture buffers are numbered from left to right,
but inside this construct the numbering is
restarted for each branch.
The numbering within each branch will be as
normal, and any buffers following this construct
will be numbered as though the construct contained
only one branch, that being the one with the most
capture buffers in it.
This construct will be useful when you want to
capture one of a number of alternative matches.
Consider the following pattern. The numbers
underneath show in which buffer the captured
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content will be stored.
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
Be careful when using the branch reset pattern in
combination with named captures. Named captures
are implemented as being aliases to numbered
buffers holding the captures, and that interferes
with the implementation of the branch reset
pattern. If you are using named captures in a
branch reset pattern, it's best to use the same
names, in the same order, in each of the
alternations:
/(?| (?<a> x ) (?<b> y )
| (?<a> z ) (?<b> w )) /x
Not doing so may lead to surprises:
"12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
say $+ {a}; # Prints '12'
say $+ {b}; # *Also* prints '12'.
The problem here is that both the buffer named "a"
and the buffer named "b" are aliases for the
buffer belonging to $1.
Look-Around Assertions
Look-around assertions are zero width patterns
which match a specific pattern without including
it in $&. Positive assertions match when their
subpattern matches, negative assertions match when
their subpattern fails. Look-behind matches text
up to the current match position, look-ahead
matches text following the current match position.
"(?=pattern)"
A zero-width positive look-ahead assertion.
For example, "/\w+(?=\t)/" matches a word
followed by a tab, without including the tab
in $&.
"(?!pattern)"
A zero-width negative look-ahead assertion.
For example "/foo(?!bar)/" matches any
occurrence of "foo" that isn't followed by
"bar". Note however that look-ahead and look-
behind are NOT the same thing. You cannot use
this for look-behind.
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If you are looking for a "bar" that isn't
preceded by a "foo", "/(?!foo)bar/" will not
do what you want. That's because the
"(?!foo)" is just saying that the next thing
cannot be "foo"--and it's not, it's a "bar",
so "foobar" will match. You would have to do
something like "/(?!foo)...bar/" for that.
We say "like" because there's the case of your
"bar" not having three characters before it.
You could cover that this way:
"/(?:(?!foo)...|^.{0,2})bar/". Sometimes it's
still easier just to say:
if (/bar/ && $` !~ /foo$/)
For look-behind see below.
"(?<=pattern)" "\K"
A zero-width positive look-behind assertion.
For example, "/(?<=\t)\w+/" matches a word
that follows a tab, without including the tab
in $&. Works only for fixed-width look-
behind.
There is a special form of this construct,
called "\K", which causes the regex engine to
"keep" everything it had matched prior to the
"\K" and not include it in $&. This
effectively provides variable length look-
behind. The use of "\K" inside of another
look-around assertion is allowed, but the
behaviour is currently not well defined.
For various reasons "\K" may be significantly
more efficient than the equivalent "(?<=...)"
construct, and it is especially useful in
situations where you want to efficiently
remove something following something else in a
string. For instance
s/(foo)bar/$1/g;
can be rewritten as the much more efficient
s/foo\Kbar//g;
"(?<!pattern)"
A zero-width negative look-behind assertion.
For example "/(?<!bar)foo/" matches any
occurrence of "foo" that does not follow
"bar". Works only for fixed-width look-
behind.
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"(?'NAME'pattern)"
"(?<NAME>pattern)"
A named capture buffer. Identical in every respect
to normal capturing parentheses "()" but for the
additional fact that "%+" or "%-" may be used
after a successful match to refer to a named
buffer. See "perlvar" for more details on the "%+"
and "%-" hashes.
If multiple distinct capture buffers have the same
name then the $+{NAME} will refer to the leftmost
defined buffer in the match.
The forms "(?'NAME'pattern)" and
"(?<NAME>pattern)" are equivalent.
NOTE: While the notation of this construct is the
same as the similar function in .NET regexes, the
behavior is not. In Perl the buffers are numbered
sequentially regardless of being named or not.
Thus in the pattern
/(x)(?<foo>y)(z)/
$+{foo} will be the same as $2, and $3 will
contain 'z' instead of the opposite which is what
a .NET regex hacker might expect.
Currently NAME is restricted to simple identifiers
only. In other words, it must match
"/^[_A-Za-z][_A-Za-z0-9]*\z/" or its Unicode
extension (see utf8), though it isn't extended by
the locale (see perllocale).
NOTE: In order to make things easier for
programmers with experience with the Python or
PCRE regex engines, the pattern
"(?P<NAME>pattern)" may be used instead of
"(?<NAME>pattern)"; however this form does not
support the use of single quotes as a delimiter
for the name.
"\k<NAME>"
"\k'NAME'"
Named backreference. Similar to numeric
backreferences, except that the group is
designated by name and not number. If multiple
groups have the same name then it refers to the
leftmost defined group in the current match.
It is an error to refer to a name not defined by a
"(?<NAME>)" earlier in the pattern.
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Both forms are equivalent.
NOTE: In order to make things easier for
programmers with experience with the Python or
PCRE regex engines, the pattern "(?P=NAME)" may be
used instead of "\k<NAME>".
"(?{ code })"
WARNING: This extended regular expression feature
is considered experimental, and may be changed
without notice. Code executed that has side
effects may not perform identically from version
to version due to the effect of future
optimisations in the regex engine.
This zero-width assertion evaluates any embedded
Perl code. It always succeeds, and its "code" is
not interpolated. Currently, the rules to
determine where the "code" ends are somewhat
convoluted.
This feature can be used together with the special
variable $^N to capture the results of submatches
in variables without having to keep track of the
number of nested parentheses. For example:
$_ = "The brown fox jumps over the lazy dog";
/the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
print "color = $color, animal = $animal\n";
Inside the "(?{...})" block, $_ refers to the
string the regular expression is matching against.
You can also use "pos()" to know what is the
current position of matching within this string.
The "code" is properly scoped in the following
sense: If the assertion is backtracked (compare
"Backtracking"), all changes introduced after
"local"ization are undone, so that
$_ = 'a' x 8;
m<
(?{ $cnt = 0 }) # Initialize $cnt.
(
a
(?{
local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
})
)*
aaaa
(?{ $res = $cnt }) # On success copy to non-localized
# location.
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>x;
will set "$res = 4". Note that after the match,
$cnt returns to the globally introduced value,
because the scopes that restrict "local" operators
are unwound.
This assertion may be used as a
"(?(condition)yes-pattern|no-pattern)" switch. If
not used in this way, the result of evaluation of
"code" is put into the special variable $^R. This
happens immediately, so $^R can be used from other
"(?{ code })" assertions inside the same regular
expression.
The assignment to $^R above is properly localized,
so the old value of $^R is restored if the
assertion is backtracked; compare "Backtracking".
For reasons of security, this construct is
forbidden if the regular expression involves run-
time interpolation of variables, unless the
perilous "use re 'eval'" pragma has been used (see
re), or the variables contain results of "qr//"
operator (see "qr/STRING/msixpo" in perlop).
This restriction is due to the wide-spread and
remarkably convenient custom of using run-time
determined strings as patterns. For example:
$re = <>;
chomp $re;
$string =~ /$re/;
Before Perl knew how to execute interpolated code
within a pattern, this operation was completely
safe from a security point of view, although it
could raise an exception from an illegal pattern.
If you turn on the "use re 'eval'", though, it is
no longer secure, so you should only do so if you
are also using taint checking. Better yet, use
the carefully constrained evaluation within a Safe
compartment. See perlsec for details about both
these mechanisms.
WARNING: Use of lexical ("my") variables in these
blocks is broken. The result is unpredictable and
will make perl unstable. The workaround is to use
global ("our") variables.
WARNING: Because Perl's regex engine is currently
not re-entrant, interpolated code may not invoke
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the regex engine either directly with "m//" or
"s///"), or indirectly with functions such as
"split". Invoking the regex engine in these blocks
will make perl unstable.
"(??{ code })"
WARNING: This extended regular expression feature
is considered experimental, and may be changed
without notice. Code executed that has side
effects may not perform identically from version
to version due to the effect of future
optimisations in the regex engine.
This is a "postponed" regular subexpression. The
"code" is evaluated at run time, at the moment
this subexpression may match. The result of
evaluation is considered as a regular expression
and matched as if it were inserted instead of this
construct. Note that this means that the contents
of capture buffers defined inside an eval'ed
pattern are not available outside of the pattern,
and vice versa, there is no way for the inner
pattern to refer to a capture buffer defined
outside. Thus,
('a' x 100)=~/(??{'(.)' x 100})/
will match, it will not set $1.
The "code" is not interpolated. As before, the
rules to determine where the "code" ends are
currently somewhat convoluted.
The following pattern matches a parenthesized
group:
$re = qr{
\(
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
(??{ $re }) # Group with matching parens
)*
\)
}x;
See also "(?PARNO)" for a different, more
efficient way to accomplish the same task.
For reasons of security, this construct is
forbidden if the regular expression involves run-
time interpolation of variables, unless the
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perilous "use re 'eval'" pragma has been used (see
re), or the variables contain results of "qr//"
operator (see "qr/STRING/msixpo" in perlop).
Because perl's regex engine is not currently re-
entrant, delayed code may not invoke the regex
engine either directly with "m//" or "s///"), or
indirectly with functions such as "split".
Recursing deeper than 50 times without consuming
any input string will result in a fatal error.
The maximum depth is compiled into perl, so
changing it requires a custom build.
"(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
Similar to "(??{ code })" except it does not
involve compiling any code, instead it treats the
contents of a capture buffer as an independent
pattern that must match at the current position.
Capture buffers contained by the pattern will have
the value as determined by the outermost
recursion.
PARNO is a sequence of digits (not starting with
0) whose value reflects the paren-number of the
capture buffer to recurse to. "(?R)" recurses to
the beginning of the whole pattern. "(?0)" is an
alternate syntax for "(?R)". If PARNO is preceded
by a plus or minus sign then it is assumed to be
relative, with negative numbers indicating
preceding capture buffers and positive ones
following. Thus "(?-1)" refers to the most
recently declared buffer, and "(?+1)" indicates
the next buffer to be declared. Note that the
counting for relative recursion differs from that
of relative backreferences, in that with recursion
unclosed buffers are included.
The following pattern matches a function foo()
which may contain balanced parentheses as the
argument.
$re = qr{ ( # paren group 1 (full function)
foo
( # paren group 2 (parens)
\(
( # paren group 3 (contents of parens)
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
(?2) # Recurse to start of paren group 2
)*
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)
\)
)
)
}x;
If the pattern was used as follows
'foo(bar(baz)+baz(bop))'=~/$re/
and print "\$1 = $1\n",
"\$2 = $2\n",
"\$3 = $3\n";
the output produced should be the following:
$1 = foo(bar(baz)+baz(bop))
$2 = (bar(baz)+baz(bop))
$3 = bar(baz)+baz(bop)
If there is no corresponding capture buffer
defined, then it is a fatal error. Recursing
deeper than 50 times without consuming any input
string will also result in a fatal error. The
maximum depth is compiled into perl, so changing
it requires a custom build.
The following shows how using negative indexing
can make it easier to embed recursive patterns
inside of a "qr//" construct for later use:
my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
if (/foo $parens \s+ + \s+ bar $parens/x) {
# do something here...
}
Note that this pattern does not behave the same
way as the equivalent PCRE or Python construct of
the same form. In Perl you can backtrack into a
recursed group, in PCRE and Python the recursed
into group is treated as atomic. Also, modifiers
are resolved at compile time, so constructs like
(?i:(?1)) or (?:(?i)(?1)) do not affect how the
sub-pattern will be processed.
"(?&NAME)"
Recurse to a named subpattern. Identical to
"(?PARNO)" except that the parenthesis to recurse
to is determined by name. If multiple parentheses
have the same name, then it recurses to the
leftmost.
It is an error to refer to a name that is not
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declared somewhere in the pattern.
NOTE: In order to make things easier for
programmers with experience with the Python or
PCRE regex engines the pattern "(?P>NAME)" may be
used instead of "(?&NAME)".
"(?(condition)yes-pattern|no-pattern)"
"(?(condition)yes-pattern)"
Conditional expression. "(condition)" should be
either an integer in parentheses (which is valid
if the corresponding pair of parentheses matched),
a look-ahead/look-behind/evaluate zero-width
assertion, a name in angle brackets or single
quotes (which is valid if a buffer with the given
name matched), or the special symbol (R) (true
when evaluated inside of recursion or eval).
Additionally the R may be followed by a number,
(which will be true when evaluated when recursing
inside of the appropriate group), or by &NAME, in
which case it will be true only when evaluated
during recursion in the named group.
Here's a summary of the possible predicates:
(1) (2) ...
Checks if the numbered capturing buffer has
matched something.
(<NAME>) ('NAME')
Checks if a buffer with the given name has
matched something.
(?{ CODE })
Treats the code block as the condition.
(R) Checks if the expression has been evaluated
inside of recursion.
(R1) (R2) ...
Checks if the expression has been evaluated
while executing directly inside of the n-th
capture group. This check is the regex
equivalent of
if ((caller(0))[3] eq 'subname') { ... }
In other words, it does not check the full
recursion stack.
(R&NAME)
Similar to "(R1)", this predicate checks to
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see if we're executing directly inside of the
leftmost group with a given name (this is the
same logic used by "(?&NAME)" to
disambiguate). It does not check the full
stack, but only the name of the innermost
active recursion.
(DEFINE)
In this case, the yes-pattern is never
directly executed, and no no-pattern is
allowed. Similar in spirit to "(?{0})" but
more efficient. See below for details.
For example:
m{ ( \( )?
[^()]+
(?(1) \) )
}x
matches a chunk of non-parentheses, possibly
included in parentheses themselves.
A special form is the "(DEFINE)" predicate, which
never executes directly its yes-pattern, and does
not allow a no-pattern. This allows to define
subpatterns which will be executed only by using
the recursion mechanism. This way, you can define
a set of regular expression rules that can be
bundled into any pattern you choose.
It is recommended that for this usage you put the
DEFINE block at the end of the pattern, and that
you name any subpatterns defined within it.
Also, it's worth noting that patterns defined this
way probably will not be as efficient, as the
optimiser is not very clever about handling them.
An example of how this might be used is as
follows:
/(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
(?(DEFINE)
(?<NAME_PAT>....)
(?<ADRESS_PAT>....)
)/x
Note that capture buffers matched inside of
recursion are not accessible after the recursion
returns, so the extra layer of capturing buffers
is necessary. Thus $+{NAME_PAT} would not be
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defined even though $+{NAME} would be.
"(?>pattern)"
An "independent" subexpression, one which matches
the substring that a standalone "pattern" would
match if anchored at the given position, and it
matches nothing other than this substring. This
construct is useful for optimizations of what
would otherwise be "eternal" matches, because it
will not backtrack (see "Backtracking"). It may
also be useful in places where the "grab all you
can, and do not give anything back" semantic is
desirable.
For example: "^(?>a*)ab" will never match, since
"(?>a*)" (anchored at the beginning of string, as
above) will match all characters "a" at the
beginning of string, leaving no "a" for "ab" to
match. In contrast, "a*ab" will match the same as
"a+b", since the match of the subgroup "a*" is
influenced by the following group "ab" (see
"Backtracking"). In particular, "a*" inside
"a*ab" will match fewer characters than a
standalone "a*", since this makes the tail match.
An effect similar to "(?>pattern)" may be achieved
by writing "(?=(pattern))\1". This matches the
same substring as a standalone "a+", and the
following "\1" eats the matched string; it
therefore makes a zero-length assertion into an
analogue of "(?>...)". (The difference between
these two constructs is that the second one uses a
capturing group, thus shifting ordinals of
backreferences in the rest of a regular
expression.)
Consider this pattern:
m{ \(
(
[^()]+ # x+
|
\( [^()]* \)
)+
\)
}x
That will efficiently match a nonempty group with
matching parentheses two levels deep or less.
However, if there is no such group, it will take
virtually forever on a long string. That's
because there are so many different ways to split
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a long string into several substrings. This is
what "(.+)+" is doing, and "(.+)+" is similar to a
subpattern of the above pattern. Consider how the
pattern above detects no-match on
"((()aaaaaaaaaaaaaaaaaa" in several seconds, but
that each extra letter doubles this time. This
exponential performance will make it appear that
your program has hung. However, a tiny change to
this pattern
m{ \(
(
(?> [^()]+ ) # change x+ above to (?> x+ )
|
\( [^()]* \)
)+
\)
}x
which uses "(?>...)" matches exactly when the one
above does (verifying this yourself would be a
productive exercise), but finishes in a fourth the
time when used on a similar string with 1000000
"a"s. Be aware, however, that this pattern
currently triggers a warning message under the
"use warnings" pragma or -w switch saying it
"matches null string many times in regex".
On simple groups, such as the pattern "(?> [^()]+
)", a comparable effect may be achieved by
negative look-ahead, as in "[^()]+ (?! [^()] )".
This was only 4 times slower on a string with
1000000 "a"s.
The "grab all you can, and do not give anything
back" semantic is desirable in many situations
where on the first sight a simple "()*" looks like
the correct solution. Suppose we parse text with
comments being delimited by "#" followed by some
optional (horizontal) whitespace. Contrary to its
appearance, "#[ \t]*" is not the correct
subexpression to match the comment delimiter,
because it may "give up" some whitespace if the
remainder of the pattern can be made to match that
way. The correct answer is either one of these:
(?>#[ \t]*)
#[ \t]*(?![ \t])
For example, to grab non-empty comments into $1,
one should use either one of these:
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/ (?> \# [ \t]* ) ( .+ ) /x;
/ \# [ \t]* ( [^ \t] .* ) /x;
Which one you pick depends on which of these
expressions better reflects the above
specification of comments.
In some literature this construct is called
"atomic matching" or "possessive matching".
Possessive quantifiers are equivalent to putting
the item they are applied to inside of one of
these constructs. The following equivalences
apply:
Quantifier Form Bracketing Form
--------------- ---------------
PAT*+ (?>PAT*)
PAT++ (?>PAT+)
PAT?+ (?>PAT?)
PAT{min,max}+ (?>PAT{min,max})
Special Backtracking Control Verbs
WARNING: These patterns are experimental and subject to
change or removal in a future version of Perl. Their usage
in production code should be noted to avoid problems during
upgrades.
These special patterns are generally of the form
"(*VERB:ARG)". Unless otherwise stated the ARG argument is
optional; in some cases, it is forbidden.
Any pattern containing a special backtracking verb that
allows an argument has the special behaviour that when
executed it sets the current package's $REGERROR and
$REGMARK variables. When doing so the following rules apply:
On failure, the $REGERROR variable will be set to the ARG
value of the verb pattern, if the verb was involved in the
failure of the match. If the ARG part of the pattern was
omitted, then $REGERROR will be set to the name of the last
"(*MARK:NAME)" pattern executed, or to TRUE if there was
none. Also, the $REGMARK variable will be set to FALSE.
On a successful match, the $REGERROR variable will be set to
FALSE, and the $REGMARK variable will be set to the name of
the last "(*MARK:NAME)" pattern executed. See the
explanation for the "(*MARK:NAME)" verb below for more
details.
NOTE: $REGERROR and $REGMARK are not magic variables like $1
and most other regex related variables. They are not local
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to a scope, nor readonly, but instead are volatile package
variables similar to $AUTOLOAD. Use "local" to localize
changes to them to a specific scope if necessary.
If a pattern does not contain a special backtracking verb
that allows an argument, then $REGERROR and $REGMARK are not
touched at all.
Verbs that take an argument
"(*PRUNE)" "(*PRUNE:NAME)"
This zero-width pattern prunes the backtracking tree
at the current point when backtracked into on
failure. Consider the pattern "A (*PRUNE) B", where
A and B are complex patterns. Until the "(*PRUNE)"
verb is reached, A may backtrack as necessary to
match. Once it is reached, matching continues in B,
which may also backtrack as necessary; however,
should B not match, then no further backtracking
will take place, and the pattern will fail outright
at the current starting position.
The following example counts all the possible
matching strings in a pattern (without actually
matching any of them).
'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
which produces:
aaab
aaa
aa
a
aab
aa
a
ab
a
Count=9
If we add a "(*PRUNE)" before the count like the
following
'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
we prevent backtracking and find the count of the
longest matching at each matching starting point
like so:
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aaab
aab
ab
Count=3
Any number of "(*PRUNE)" assertions may be used in a
pattern.
See also "(?>pattern)" and possessive quantifiers
for other ways to control backtracking. In some
cases, the use of "(*PRUNE)" can be replaced with a
"(?>pattern)" with no functional difference;
however, "(*PRUNE)" can be used to handle cases that
cannot be expressed using a "(?>pattern)" alone.
"(*SKIP)" "(*SKIP:NAME)"
This zero-width pattern is similar to "(*PRUNE)",
except that on failure it also signifies that
whatever text that was matched leading up to the
"(*SKIP)" pattern being executed cannot be part of
any match of this pattern. This effectively means
that the regex engine "skips" forward to this
position on failure and tries to match again,
(assuming that there is sufficient room to match).
The name of the "(*SKIP:NAME)" pattern has special
significance. If a "(*MARK:NAME)" was encountered
while matching, then it is that position which is
used as the "skip point". If no "(*MARK)" of that
name was encountered, then the "(*SKIP)" operator
has no effect. When used without a name the "skip
point" is where the match point was when executing
the (*SKIP) pattern.
Compare the following to the examples in "(*PRUNE)",
note the string is twice as long:
'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
outputs
aaab
aaab
Count=2
Once the 'aaab' at the start of the string has
matched, and the "(*SKIP)" executed, the next
starting point will be where the cursor was when the
"(*SKIP)" was executed.
"(*MARK:NAME)" "(*:NAME)" "(*MARK:NAME)" "(*:NAME)"
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This zero-width pattern can be used to mark the
point reached in a string when a certain part of the
pattern has been successfully matched. This mark may
be given a name. A later "(*SKIP)" pattern will then
skip forward to that point if backtracked into on
failure. Any number of "(*MARK)" patterns are
allowed, and the NAME portion may be duplicated.
In addition to interacting with the "(*SKIP)"
pattern, "(*MARK:NAME)" can be used to "label" a
pattern branch, so that after matching, the program
can determine which branches of the pattern were
involved in the match.
When a match is successful, the $REGMARK variable
will be set to the name of the most recently
executed "(*MARK:NAME)" that was involved in the
match.
This can be used to determine which branch of a
pattern was matched without using a separate capture
buffer for each branch, which in turn can result in
a performance improvement, as perl cannot optimize
"/(?:(x)|(y)|(z))/" as efficiently as something like
"/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/".
When a match has failed, and unless another verb has
been involved in failing the match and has provided
its own name to use, the $REGERROR variable will be
set to the name of the most recently executed
"(*MARK:NAME)".
See "(*SKIP)" for more details.
As a shortcut "(*MARK:NAME)" can be written
"(*:NAME)".
"(*THEN)" "(*THEN:NAME)"
This is similar to the "cut group" operator "::"
from Perl 6. Like "(*PRUNE)", this verb always
matches, and when backtracked into on failure, it
causes the regex engine to try the next alternation
in the innermost enclosing group (capturing or
otherwise).
Its name comes from the observation that this
operation combined with the alternation operator
("|") can be used to create what is essentially a
pattern-based if/then/else block:
( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
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Note that if this operator is used and NOT inside of
an alternation then it acts exactly like the
"(*PRUNE)" operator.
/ A (*PRUNE) B /
is the same as
/ A (*THEN) B /
but
/ ( A (*THEN) B | C (*THEN) D ) /
is not the same as
/ ( A (*PRUNE) B | C (*PRUNE) D ) /
as after matching the A but failing on the B the
"(*THEN)" verb will backtrack and try C; but the
"(*PRUNE)" verb will simply fail.
"(*COMMIT)"
This is the Perl 6 "commit pattern" "<commit>" or
":::". It's a zero-width pattern similar to
"(*SKIP)", except that when backtracked into on
failure it causes the match to fail outright. No
further attempts to find a valid match by advancing
the start pointer will occur again. For example,
'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
outputs
aaab
Count=1
In other words, once the "(*COMMIT)" has been
entered, and if the pattern does not match, the
regex engine will not try any further matching on
the rest of the string.
Verbs without an argument
"(*FAIL)" "(*F)"
This pattern matches nothing and always fails. It
can be used to force the engine to backtrack. It is
equivalent to "(?!)", but easier to read. In fact,
"(?!)" gets optimised into "(*FAIL)" internally.
It is probably useful only when combined with
"(?{})" or "(??{})".
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"(*ACCEPT)"
WARNING: This feature is highly experimental. It is
not recommended for production code.
This pattern matches nothing and causes the end of
successful matching at the point at which the
"(*ACCEPT)" pattern was encountered, regardless of
whether there is actually more to match in the
string. When inside of a nested pattern, such as
recursion, or in a subpattern dynamically generated
via "(??{})", only the innermost pattern is ended
immediately.
If the "(*ACCEPT)" is inside of capturing buffers
then the buffers are marked as ended at the point at
which the "(*ACCEPT)" was encountered. For
instance:
'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
will match, and $1 will be "AB" and $2 will be "B",
$3 will not be set. If another branch in the inner
parentheses were matched, such as in the string
'ACDE', then the "D" and "E" would have to be
matched as well.
Backtracking
NOTE: This section presents an abstract approximation of
regular expression behavior. For a more rigorous (and
complicated) view of the rules involved in selecting a match
among possible alternatives, see "Combining RE Pieces".
A fundamental feature of regular expression matching
involves the notion called backtracking, which is currently
used (when needed) by all regular non-possessive expression
quantifiers, namely "*", "*?", "+", "+?", "{n,m}", and
"{n,m}?". Backtracking is often optimized internally, but
the general principle outlined here is valid.
For a regular expression to match, the entire regular
expression must match, not just part of it. So if the
beginning of a pattern containing a quantifier succeeds in a
way that causes later parts in the pattern to fail, the
matching engine backs up and recalculates the beginning
part--that's why it's called backtracking.
Here is an example of backtracking: Let's say you want to
find the word following "foo" in the string "Food is on the
foo table.":
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$_ = "Food is on the foo table.";
if ( /\b(foo)\s+(\w+)/i ) {
print "$2 follows $1.\n";
}
When the match runs, the first part of the regular
expression ("\b(foo)") finds a possible match right at the
beginning of the string, and loads up $1 with "Foo".
However, as soon as the matching engine sees that there's no
whitespace following the "Foo" that it had saved in $1, it
realizes its mistake and starts over again one character
after where it had the tentative match. This time it goes
all the way until the next occurrence of "foo". The complete
regular expression matches this time, and you get the
expected output of "table follows foo."
Sometimes minimal matching can help a lot. Imagine you'd
like to match everything between "foo" and "bar".
Initially, you write something like this:
$_ = "The food is under the bar in the barn.";
if ( /foo(.*)bar/ ) {
print "got <$1>\n";
}
Which perhaps unexpectedly yields:
got <d is under the bar in the >
That's because ".*" was greedy, so you get everything
between the first "foo" and the last "bar". Here it's more
effective to use minimal matching to make sure you get the
text between a "foo" and the first "bar" thereafter.
if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
got <d is under the >
Here's another example. Let's say you'd like to match a
number at the end of a string, and you also want to keep the
preceding part of the match. So you write this:
$_ = "I have 2 numbers: 53147";
if ( /(.*)(\d*)/ ) { # Wrong!
print "Beginning is <$1>, number is <$2>.\n";
}
That won't work at all, because ".*" was greedy and gobbled
up the whole string. As "\d*" can match on an empty string
the complete regular expression matched successfully.
Beginning is <I have 2 numbers: 53147>, number is <>.
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Here are some variants, most of which don't work:
$_ = "I have 2 numbers: 53147";
@pats = qw{
(.*)(\d*)
(.*)(\d+)
(.*?)(\d*)
(.*?)(\d+)
(.*)(\d+)$
(.*?)(\d+)$
(.*)\b(\d+)$
(.*\D)(\d+)$
};
for $pat (@pats) {
printf "%-12s ", $pat;
if ( /$pat/ ) {
print "<$1> <$2>\n";
} else {
print "FAIL\n";
}
}
That will print out:
(.*)(\d*) <I have 2 numbers: 53147> <>
(.*)(\d+) <I have 2 numbers: 5314> <7>
(.*?)(\d*) <> <>
(.*?)(\d+) <I have > <2>
(.*)(\d+)$ <I have 2 numbers: 5314> <7>
(.*?)(\d+)$ <I have 2 numbers: > <53147>
(.*)\b(\d+)$ <I have 2 numbers: > <53147>
(.*\D)(\d+)$ <I have 2 numbers: > <53147>
As you see, this can be a bit tricky. It's important to
realize that a regular expression is merely a set of
assertions that gives a definition of success. There may be
0, 1, or several different ways that the definition might
succeed against a particular string. And if there are
multiple ways it might succeed, you need to understand
backtracking to know which variety of success you will
achieve.
When using look-ahead assertions and negations, this can all
get even trickier. Imagine you'd like to find a sequence of
non-digits not followed by "123". You might try to write
that as
$_ = "ABC123";
if ( /^\D*(?!123)/ ) { # Wrong!
print "Yup, no 123 in $_\n";
}
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But that isn't going to match; at least, not the way you're
hoping. It claims that there is no 123 in the string.
Here's a clearer picture of why that pattern matches,
contrary to popular expectations:
$x = 'ABC123';
$y = 'ABC445';
print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;
print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;
This prints
2: got ABC
3: got AB
4: got ABC
You might have expected test 3 to fail because it seems to a
more general purpose version of test 1. The important
difference between them is that test 3 contains a quantifier
("\D*") and so can use backtracking, whereas test 1 will
not. What's happening is that you've asked "Is it true that
at the start of $x, following 0 or more non-digits, you have
something that's not 123?" If the pattern matcher had let
"\D*" expand to "ABC", this would have caused the whole
pattern to fail.
The search engine will initially match "\D*" with "ABC".
Then it will try to match "(?!123" with "123", which fails.
But because a quantifier ("\D*") has been used in the
regular expression, the search engine can backtrack and
retry the match differently in the hope of matching the
complete regular expression.
The pattern really, really wants to succeed, so it uses the
standard pattern back-off-and-retry and lets "\D*" expand to
just "AB" this time. Now there's indeed something following
"AB" that is not "123". It's "C123", which suffices.
We can deal with this by using both an assertion and a
negation. We'll say that the first part in $1 must be
followed both by a digit and by something that's not "123".
Remember that the look-aheads are zero-width
expressions--they only look, but don't consume any of the
string in their match. So rewriting this way produces what
you'd expect; that is, case 5 will fail, but case 6
succeeds:
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print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;
6: got ABC
In other words, the two zero-width assertions next to each
other work as though they're ANDed together, just as you'd
use any built-in assertions: "/^$/" matches only if you're
at the beginning of the line AND the end of the line
simultaneously. The deeper underlying truth is that
juxtaposition in regular expressions always means AND,
except when you write an explicit OR using the vertical bar.
"/ab/" means match "a" AND (then) match "b", although the
attempted matches are made at different positions because
"a" is not a zero-width assertion, but a one-width
assertion.
WARNING: Particularly complicated regular expressions can
take exponential time to solve because of the immense number
of possible ways they can use backtracking to try for a
match. For example, without internal optimizations done by
the regular expression engine, this will take a painfully
long time to run:
'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
And if you used "*"'s in the internal groups instead of
limiting them to 0 through 5 matches, then it would take
forever--or until you ran out of stack space. Moreover,
these internal optimizations are not always applicable. For
example, if you put "{0,5}" instead of "*" on the external
group, no current optimization is applicable, and the match
takes a long time to finish.
A powerful tool for optimizing such beasts is what is known
as an "independent group", which does not backtrack (see
"(?>pattern)"). Note also that zero-length
look-ahead/look-behind assertions will not backtrack to make
the tail match, since they are in "logical" context: only
whether they match is considered relevant. For an example
where side-effects of look-ahead might have influenced the
following match, see "(?>pattern)".
Version 8 Regular Expressions
In case you're not familiar with the "regular" Version 8
regex routines, here are the pattern-matching rules not
described above.
Any single character matches itself, unless it is a
metacharacter with a special meaning described here or
above. You can cause characters that normally function as
metacharacters to be interpreted literally by prefixing them
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with a "\" (e.g., "\." matches a ".", not any character;
"\\" matches a "\"). This escape mechanism is also required
for the character used as the pattern delimiter.
A series of characters matches that series of characters in
the target string, so the pattern "blurfl" would match
"blurfl" in the target string.
You can specify a character class, by enclosing a list of
characters in "[]", which will match any character from the
list. If the first character after the "[" is "^", the
class matches any character not in the list. Within a list,
the "-" character specifies a range, so that "a-z"
represents all characters between "a" and "z", inclusive.
If you want either "-" or "]" itself to be a member of a
class, put it at the start of the list (possibly after a
"^"), or escape it with a backslash. "-" is also taken
literally when it is at the end of the list, just before the
closing "]". (The following all specify the same class of
three characters: "[-az]", "[az-]", and "[a\-z]". All are
different from "[a-z]", which specifies a class containing
twenty-six characters, even on EBCDIC-based character sets.)
Also, if you try to use the character classes "\w", "\W",
"\s", "\S", "\d", or "\D" as endpoints of a range, the "-"
is understood literally.
Note also that the whole range idea is rather unportable
between character sets--and even within character sets they
may cause results you probably didn't expect. A sound
principle is to use only ranges that begin from and end at
either alphabetics of equal case ([a-e], [A-E]), or digits
([0-9]). Anything else is unsafe. If in doubt, spell out
the character sets in full.
Characters may be specified using a metacharacter syntax
much like that used in C: "\n" matches a newline, "\t" a
tab, "\r" a carriage return, "\f" a form feed, etc. More
generally, \nnn, where nnn is a string of octal digits,
matches the character whose coded character set value is
nnn. Similarly, \xnn, where nn are hexadecimal digits,
matches the character whose numeric value is nn. The
expression \cx matches the character control-x. Finally,
the "." metacharacter matches any character except "\n"
(unless you use "/s").
You can specify a series of alternatives for a pattern using
"|" to separate them, so that "fee|fie|foe" will match any
of "fee", "fie", or "foe" in the target string (as would
"f(e|i|o)e"). The first alternative includes everything
from the last pattern delimiter ("(", "[", or the beginning
of the pattern) up to the first "|", and the last
alternative contains everything from the last "|" to the
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next pattern delimiter. That's why it's common practice to
include alternatives in parentheses: to minimize confusion
about where they start and end.
Alternatives are tried from left to right, so the first
alternative found for which the entire expression matches,
is the one that is chosen. This means that alternatives are
not necessarily greedy. For example: when matching
"foo|foot" against "barefoot", only the "foo" part will
match, as that is the first alternative tried, and it
successfully matches the target string. (This might not seem
important, but it is important when you are capturing
matched text using parentheses.)
Also remember that "|" is interpreted as a literal within
square brackets, so if you write "[fee|fie|foe]" you're
really only matching "[feio|]".
Within a pattern, you may designate subpatterns for later
reference by enclosing them in parentheses, and you may
refer back to the nth subpattern later in the pattern using
the metacharacter \n. Subpatterns are numbered based on the
left to right order of their opening parenthesis. A
backreference matches whatever actually matched the
subpattern in the string being examined, not the rules for
that subpattern. Therefore, "(0|0x)\d*\s\1\d*" will match
"0x1234 0x4321", but not "0x1234 01234", because subpattern
1 matched "0x", even though the rule "0|0x" could
potentially match the leading 0 in the second number.
Warning on \1 Instead of $1
Some people get too used to writing things like:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered (for \1 to \9) for the RHS of a
substitute to avoid shocking the sed addicts, but it's a
dirty habit to get into. That's because in PerlThink, the
righthand side of an "s///" is a double-quoted string. "\1"
in the usual double-quoted string means a control-A. The
customary Unix meaning of "\1" is kludged in for "s///".
However, if you get into the habit of doing that, you get
yourself into trouble if you then add an "/e" modifier.
s/(\d+)/ \1 + 1 /eg; # causes warning under -w
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying "\{1}000", whereas you
can fix it with "${1}000". The operation of interpolation
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should not be confused with the operation of matching a
backreference. Certainly they mean two different things on
the left side of the "s///".
Repeated Patterns Matching a Zero-length Substring
WARNING: Difficult material (and prose) ahead. This section
needs a rewrite.
Regular expressions provide a terse and powerful programming
language. As with most other power tools, power comes
together with the ability to wreak havoc.
A common abuse of this power stems from the ability to make
infinite loops using regular expressions, with something as
innocuous as:
'foo' =~ m{ ( o? )* }x;
The "o?" matches at the beginning of 'foo', and since the
position in the string is not moved by the match, "o?" would
match again and again because of the "*" quantifier.
Another common way to create a similar cycle is with the
looping modifier "//g":
@matches = ( 'foo' =~ m{ o? }xg );
or
print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
or the loop implied by split().
However, long experience has shown that many programming
tasks may be significantly simplified by using repeated
subexpressions that may match zero-length substrings.
Here's a simple example being:
@chars = split //, $string; # // is not magic in split
($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
Thus Perl allows such constructs, by forcefully breaking the
infinite loop. The rules for this are different for lower-
level loops given by the greedy quantifiers "*+{}", and for
higher-level ones like the "/g" modifier or split()
operator.
The lower-level loops are interrupted (that is, the loop is
broken) when Perl detects that a repeated expression matched
a zero-length substring. Thus
m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
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is made equivalent to
m{ (?: NON_ZERO_LENGTH )*
|
(?: ZERO_LENGTH )?
}x;
The higher level-loops preserve an additional state between
iterations: whether the last match was zero-length. To
break the loop, the following match after a zero-length
match is prohibited to have a length of zero. This
prohibition interacts with backtracking (see
"Backtracking"), and so the second best match is chosen if
the best match is of zero length.
For example:
$_ = 'bar';
s/\w??/<$&>/g;
results in "<><b><><a><><r><>". At each position of the
string the best match given by non-greedy "??" is the zero-
length match, and the second best match is what is matched
by "\w". Thus zero-length matches alternate with one-
character-long matches.
Similarly, for repeated "m/()/g" the second-best match is
the match at the position one notch further in the string.
The additional state of being matched with zero-length is
associated with the matched string, and is reset by each
assignment to pos(). Zero-length matches at the end of the
previous match are ignored during "split".
Combining RE Pieces
Each of the elementary pieces of regular expressions which
were described before (such as "ab" or "\Z") could match at
most one substring at the given position of the input
string. However, in a typical regular expression these
elementary pieces are combined into more complicated
patterns using combining operators "ST", "S|T", "S*" etc (in
these examples "S" and "T" are regular subexpressions).
Such combinations can include alternatives, leading to a
problem of choice: if we match a regular expression "a|ab"
against "abc", will it match substring "a" or "ab"? One way
to describe which substring is actually matched is the
concept of backtracking (see "Backtracking"). However, this
description is too low-level and makes you think in terms of
a particular implementation.
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Another description starts with notions of "better"/"worse".
All the substrings which may be matched by the given regular
expression can be sorted from the "best" match to the
"worst" match, and it is the "best" match which is chosen.
This substitutes the question of "what is chosen?" by the
question of "which matches are better, and which are
worse?".
Again, for elementary pieces there is no such question,
since at most one match at a given position is possible.
This section describes the notion of better/worse for
combining operators. In the description below "S" and "T"
are regular subexpressions.
"ST"
Consider two possible matches, "AB" and "A'B'", "A" and
"A'" are substrings which can be matched by "S", "B" and
"B'" are substrings which can be matched by "T".
If "A" is better match for "S" than "A'", "AB" is a
better match than "A'B'".
If "A" and "A'" coincide: "AB" is a better match than
"AB'" if "B" is better match for "T" than "B'".
"S|T"
When "S" can match, it is a better match than when only
"T" can match.
Ordering of two matches for "S" is the same as for "S".
Similar for two matches for "T".
"S{REPEAT_COUNT}"
Matches as "SSS...S" (repeated as many times as
necessary).
"S{min,max}"
Matches as "S{max}|S{max-1}|...|S{min+1}|S{min}".
"S{min,max}?"
Matches as "S{min}|S{min+1}|...|S{max-1}|S{max}".
"S?", "S*", "S+"
Same as "S{0,1}", "S{0,BIG_NUMBER}", "S{1,BIG_NUMBER}"
respectively.
"S??", "S*?", "S+?"
Same as "S{0,1}?", "S{0,BIG_NUMBER}?",
"S{1,BIG_NUMBER}?" respectively.
"(?>S)"
Matches the best match for "S" and only that.
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"(?=S)", "(?<=S)"
Only the best match for "S" is considered. (This is
important only if "S" has capturing parentheses, and
backreferences are used somewhere else in the whole
regular expression.)
"(?!S)", "(?<!S)"
For this grouping operator there is no need to describe
the ordering, since only whether or not "S" can match is
important.
"(??{ EXPR })", "(?PARNO)"
The ordering is the same as for the regular expression
which is the result of EXPR, or the pattern contained by
capture buffer PARNO.
"(?(condition)yes-pattern|no-pattern)"
Recall that which of "yes-pattern" or "no-pattern"
actually matches is already determined. The ordering of
the matches is the same as for the chosen subexpression.
The above recipes describe the ordering of matches at a
given position. One more rule is needed to understand how a
match is determined for the whole regular expression: a
match at an earlier position is always better than a match
at a later position.
Creating Custom RE Engines
Overloaded constants (see overload) provide a simple way to
extend the functionality of the RE engine.
Suppose that we want to enable a new RE escape-sequence
"\Y|" which matches at a boundary between whitespace
characters and non-whitespace characters. Note that
"(?=\S)(?<!\S)|(?!\S)(?<=\S)" matches exactly at these
positions, so we want to have each "\Y|" in the place of the
more complicated version. We can create a module "customre"
to do this:
package customre;
use overload;
sub import {
shift;
die "No argument to customre::import allowed" if @_;
overload::constant 'qr' => \&convert;
}
sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}
# We must also take care of not escaping the legitimate \\Y|
# sequence, hence the presence of '\\' in the conversion rules.
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my %rules = ( '\\' => '\\\\',
'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
sub convert {
my $re = shift;
$re =~ s{
\\ ( \\ | Y . )
}
{ $rules{$1} or invalid($re,$1) }sgex;
return $re;
}
Now "use customre" enables the new escape in constant
regular expressions, i.e., those without any runtime
variable interpolations. As documented in overload, this
conversion will work only over literal parts of regular
expressions. For "\Y|$re\Y|" the variable part of this
regular expression needs to be converted explicitly (but
only if the special meaning of "\Y|" should be enabled
inside $re):
use customre;
$re = <>;
chomp $re;
$re = customre::convert $re;
/\Y|$re\Y|/;
PCRE/Python Support
As of Perl 5.10.0, Perl supports several Python/PCRE
specific extensions to the regex syntax. While Perl
programmers are encouraged to use the Perl specific syntax,
the following are also accepted:
"(?P<NAME>pattern)"
Define a named capture buffer. Equivalent to
"(?<NAME>pattern)".
"(?P=NAME)"
Backreference to a named capture buffer. Equivalent to
"\g{NAME}".
"(?P>NAME)"
Subroutine call to a named capture buffer. Equivalent to
"(?&NAME)".
BUGS
There are numerous problems with case insensitive matching
of characters outside the ASCII range, especially with those
whose folds are multiple characters, such as ligatures like
"LATIN SMALL LIGATURE FF".
In a bracketed character class with case insensitive
matching, ranges only work for ASCII characters. For
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example, "m/[\N{CYRILLIC CAPITAL LETTER A}-\N{CYRILLIC
CAPITAL LETTER YA}]/i" doesn't match all the Russian upper
and lower case letters.
Many regular expression constructs don't work on EBCDIC
platforms.
This document varies from difficult to understand to
completely and utterly opaque. The wandering prose riddled
with jargon is hard to fathom in several places.
This document needs a rewrite that separates the tutorial
content from the reference content.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
perlrequick.
perlretut.
"Regexp Quote-Like Operators" in perlop.
"Gory details of parsing quoted constructs" in perlop.
perlfaq6.
"pos" in perlfunc.
perllocale.
perlebcdic.
Mastering Regular Expressions by Jeffrey Friedl, published
by O'Reilly and Associates.
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
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http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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