ChorusOS man pages section 3STDC: Standard C Library Functions

regexec(3STDC)

NAME

regex, regcomp, regexec, regerror, regfree- regular-expression library

SYNOPSIS

#include <sys/types.h>
#include <regex.h>

preg

pattern

cflags

preg

string

nmatch

match

eflags

errcode

preg

errbuf

errbuf_size

preg

API RESTRICTIONS

The function or functions documented here may not be used safely in all application contexts with all APIs provided in the ChorusOS 5.0 product.

See API(5FEA) for details.

DESCRIPTION

These routines implement POSIX 1003.2 regular expressions (``RE''s); see the SEE ALSO section below. The regcomp() function compiles an RE written as a string into an internal form, regexec() matches that internal form against a string and reports results, regerror() transforms error codes from either into human-readable messages, and regfree() frees any dynamically-allocated storage used by the internal form of an RE.

The header <regex.h> declares two structure types, regex_t and regmatch_t , the former for compiled internal forms and the latter for match reporting. It also declares the four functions, a type regoff_t , and a number of constants with names starting with `` REG_ ''.

The regcomp() function compiles the regular expression contained in the pattern string, subject to the flags in cflags , and places the results in the regex_t structure pointed to by preg . The cflags parameter is the bitwise OR of zero or one or more of the following flags:

REG_EXTENDED: Compile modern (``extended'') REs, rather than the obsolete (``basic'') REs that are the default.
REG_BASIC: This is a synonym for 0, provided as a counterpart to REG_EXTENDED to improve readability.
REG_NOSPEC: Compile with recognition of all special characters turned off. All characters are thus considered ordinary, so the ``RE'' is a literal string. This is an extension, compatible with but not specified by POSIX 1003.2, and should be used with caution in software intended to be portable to other systems. REG_EXTENDED and REG_NOSPEC may not be used in the same call to regcomp() .
REG_ICASE: Compile for matching that ignores upper/lower case distinctions. See the citation in the SEE ALSO section below
REG_NOSUB: Compile for matching that need only report success or failure, not what was matched.
REG_NEWLINE: Compile for newline-sensitive matching. By default, newline is a completely ordinary character with no special meaning in either REs or strings. With this flag, ` [^ ' bracket expressions and ` . ' never match newline, a ` ^ ' anchor matches the null string after any newline in the string in addition to its normal function, and the ` $ ' anchor matches the null string before any newline in the string in addition to its normal function.
REG_PEND: The regular expression ends, not at the first NULL , but just before the character pointed to by the re_endp member of the structure pointed to by preg . The re_endp member is of the type const char * . This flag permits inclusion of NUL s in the RE; they are considered ordinary characters. This is an extension, compatible with, but not specified by POSIX 1003.2, and should be used with caution in software intended to be portable to other systems.

When successful, regcomp() returns 0 and fills in the structure pointed to by preg . One member of that structure (other than re_endp ) is published: re_nsub , of type size_t , contains the number of parenthesized subexpressions within the RE (except that the value of this member is undefined if the REG_NOSUB flag was used). If regcomp() fails, it returns a non-zero error code; see DIAGNOSTICS.

The regexec() function matches the compiled RE pointed to by preg against the string , subject to the flags in eflags , and reports results using nmatch , pmatch , and the returned value. The RE must have been compiled using a previous invocation of regcomp() . The compiled form is not altered during execution of regexec() , a single compiled RE can therefore be used simultaneously by multiple threads.

By default, the NUL -terminated string pointed to by string is considered to be the text of an entire line, minus any terminating newline. The eflags argument is the bitwise OR of zero or one or more of the following flags:

REG_NOTBOL: The first character of the string is not the beginning of a line, so the `^' anchor should not match before it. This does not affect the behavior of newlines under REG_NEWLINE .
REG_NOTEOL: The NUL terminating the string does not end a line, so the ` $ ' anchor should not match before it. This does not affect the behavior of newlines under REG_NEWLINE .
REG_STARTEND: The string is considered to start at string + pmatch [0]. rm_so and to have a terminating NUL located at string + pmatch [0]. rm_eo (there need not actually be a NUL at that location), regardless of the value of nmatch . See below for the definition of pmatch and nmatch . This is an extension, compatible with, but not specified by, POSIX 1003.2, and should be used with caution in software intended to be portable to other systems. Note that a non-zero rm_so does not imply REG_NOTBOL ; REG_STARTEND affects only the location of the string, not how it is matched.

See the citation in the SEE ALSO SECTION for an explanation of what is matched in situations where an RE or a portion thereof could match any of several substrings of string .

Normally, regexec() returns 0 for success and the non-zero code REG_NOMATCH for failure. Other non-zero error codes may be returned in exceptional situations; see DIAGNOSTICS.

If REG_NOSUB was specified in the compilation of the RE, or if nmatch is 0, regexec() ignores the pmatch argument (see below for the case where REG_STARTEND is specified). Otherwise, pmatch points to an array of nmatch structures of the type regmatch_t . This structure has at least the members rm_so and rm_eo , both of type regoff_t (a signed arithmetic type at least as large as an off_t and a ssize_t ), containing respectively the offset of the first character of a substring and the offset of the first character after the end of the substring. Offsets are measured from the beginning of the string argument given to regexec() . An empty substring is denoted by equal offsets, both indicating the character following the empty substring.

The 0th member of the pmatch array is filled in to indicate what substring of string was matched by the entire RE. Remaining members report what substring was matched by parenthesized subexpressions within the RE; member i reports subexpression i , with subexpressions counted (starting at 1) by the order of their opening parentheses in the RE, left to right. Unused entries in the array -- corresponding either to subexpressions that did not participate in the match at all, or to subexpressions that do not exist in the RE (that is, i > preg -> re_nsub )--have both rm_so and rm_eo set to -1. If a subexpression participated in the match several times, the substring reported is the last one it matched. (Note that, when the RE ` (b*)+ ' matches ` bbb ', the parenthesized subexpression matches each of the three `b's and then an infinite number of empty strings following the last `b', the substring reported is therefore empty.)

If REG_STARTEND is specified, pmatch must point to at least one regmatch_t (even if nmatch is 0 or REG_NOSUB was specified), to hold the input offsets for REG_STARTEND . Use for output is still entirely controlled by nmatch ; if nmatch is 0 or REG_NOSUB was specified, the value of pmatch [0] will not be changed by a successful regexec() .

The regerror() function maps a non-zero errcode from either regcomp() or regexec() to a human-readable, printable message. If preg is non- NULL , the error code should have arisen from use of the regex_t pointed to by preg , and if the error code came from regcomp() , it should have been the result of the most recent regcomp() using that regex_t . ( regerror() may be able to supply a more detailed message using information from the regex_t .) The regerror() function places the NUL -terminated message into the buffer pointed to by errbuf , limiting the length (including the NUL ) to at most errbuf_size bytes. If the whole message won't fit, as much of it as will fit before the terminating NUL is supplied. The value returned is the size of buffer needed to hold the whole message (including the terminating NULL ). If errbuf_size is 0, errbuf is ignored but the return value is still correct.

If the errcode given to regerror() is applied an OR with REG_ITOA , the ``message'' that results is the printable name of the error code, for example, `` REG_NOMATCH '', rather than an explanation of it. If errcode is REG_ATOI , preg will be non- NULL and the re_endp member of the structure it points to must point to the printable name of an error code; in this case, the result in errbuf is the decimal digits of the numeric value of the error code (0 if the name is not recognized). The REG_ITOA and REG_ATOI functions are intended primarily as debugging facilities; they are extensions, compatible with, but not specified by, POSIX 1003.2, and should be used with caution in software intended to be portable to other systems. Be warned also that they are considered experimental and changes are possible.

The regfree() function frees any dynamically-allocated storage associated with the compiled RE pointed to by preg . The remaining regex_t is no longer a valid compiled RE and the effect of supplying it to regexec() or regerror() is undefined.

None of these functions references global variables except for tables of constants; all are safe for use by multiple threads if the arguments are safe.

IMPLEMENTATION CHOICES

There are a number of decisions that 1003.2 leaves up to the implementor, either by saying ``undefined'' explicitly or by virtue of them being forbidden by the RE grammar. This implementation treats them as follows:

See the citation in the SEE ALSO section for an explanation of the definition of case-independent matching.

There is no particular limit to the length of REs, apart from memory limitations. Memory usage is approximately linear in RE size, and largely insensitive to RE complexity, except for bounded repetitions. See BUGS for one short RE using them that will run almost any system out of memory.

A backslashed character other than one specifically given a magic meaning by 1003.2 (such magic meanings occur only in obsolete [``basic''] REs) is taken as an ordinary character.

Any unmatched [ is a REG_EBRACK error.

Equivalence classes cannot begin or end bracket-expression ranges. The endpoint of one range cannot begin another.

The RE_DUP_MAX option defines the limit on repetition counts in bounded repetitions, the maximum is 255.

A repetition operator ( ? , * , + , or bounds) cannot follow another repetition operator. A repetition operator cannot begin an expression or subexpression or follow ` ^ ' or ` | '.

The pipe symbol. (` | ') cannot appear first or last in a (sub)expression or after another ` | '; in other words, an operand of ` | ' cannot be an empty subexpression. An empty parenthesized subexpression, ` () ', is legal and matches an empty (sub)string. An empty string is not a legal RE.

A brace (" { ") followed by a digit is considered the beginning of bounds for a bounded repetition, which must then follow the syntax for bounds. A ` { ' not followed by a digit is considered an ordinary character.

A circumflex (" ^ ") and dollar sign (" $ ") beginning and ending subexpressions in obsolete (``basic'') REs are anchors, not ordinary characters.

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Evolving

DIAGNOSTICS

Non-zero error codes from regcomp() and regexec() include the following:

REG_NOMATCH	regexec() failed to match
REG_BADPAT	invalid regular expression
REG_ECOLLATE	invalid collating element
REG_ECTYPE	invalid character class
REG_EESCAPE	\\ applied to unescapable character
REG_ESUBREG	invalid backreference number
REG_EBRACK	brackets [ ] not balanced
REG_EPAREN	parentheses ( ) not balanced
REG_EBRACE	braces { } not balanced
REG_BADBR	invalid repetition count(s) in { }
REG_ERANGE	invalid character range in [ ]
REG_ESPACE	ran out of memory
REG_BADRPT	?, *, or + operand invalid
REG_EMPTY	empty (sub)expression
REG_ASSERT	``can't happen''--you found a bug
REG_INVARG	invalid argument, for example,  negative-length string

BUGS

This is an alpha release with known defects. Please report problems.

There is one known functionality bug. The implementation of internationalization is incomplete: the locale is always assumed to be the default one of 1003.2, therefore, only information pertaining to that locale is available.

The back-reference code is subtle and there are doubts about its correctness in complex cases.

The regexec() function's performance is poor. This will improve with later releases. An nmatch exceeding 0 is expensive; nmatch exceeding 1 is worse. The regexec() function is largely insensitive to RE complexity except that back references are extremely expensive. RE length does matter; in particular, there is an apprecialbe speed bonus for keeping RE length under approximately 30 characters, most special characters are worth roughly double.

The regcomp() function implements bounded repetitions using expansion, which is costly in time and space if counts are large or bounded repetitions are nested. An RE like the following, ((((a{1,100}){1,100}){1,100}' will (eventually) run almost any existing machine out of swap space.

There are suspected problems with responses to obscure error conditions. Notably, certain kinds of internal overflow, produced only by extremely large REs or by multiply--nested bounded repetitions, are probably not handled well.

Due to a mistake in 1003.2, things like `a) b ' are legal REs because ` ) ' is a special character only in the presence of a previous unmatched `('. This can't be fixed until the spec is fixed.

The standard's definition of back references is vague. For example, does ` a\$\\(b\$*\\2\\)*d ' match ` abbbd '? Until the standard is clarified, behavior in such cases should not be relied on.

The implementation of word-boundary matching is imprecise, and bugs may lurk in combinations of word-boundary matching and anchoring.

RESTRICTIONS

As part of the BSD library, this function is not thread-safe.

ChorusOS 5.0 Last Revised December 2001