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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



regcmp, regex - compile and execute regular expression


#include <libgen.h>

char *regcmp(const char *string1, /* char *string2 */ ...,
     int /*(char*)0*/);
char *regex(const char *re, const char *subject,
     /* char *ret0 */ ...);

extern char *__loc1;


The regcmp() function compiles a regular expression (consisting of the concatenated arguments) and returns a pointer to the compiled form. The malloc(3C) function is used to create space for the compiled form. It is the user's responsibility to free unneeded space so allocated. A NULL return from regcmp() indicates an incorrect argument. regcmp(1) has been written to generally preclude the need for this routine at execution time.

The regex() function executes a compiled pattern against the subject string. Additional arguments are passed to receive values back. The regex() function returns NULL on failure or a pointer to the next unmatched character on success. A global character pointer __loc1 points to where the match began. The regcmp() and regex() functions were mostly borrowed from the editor ed(1); however, the syntax and semantics have been changed slightly. The following are the valid symbols and associated meanings.

[ ] * .^

This group of symbols retains its meaning as described on the regexp(7) manual page.


Matches the end of the string; \n matches a newline.

Within brackets the minus means through. For example, [a−z] is equivalent to [abcd . . .xyz]. The can appear as itself only if used as the first or last character. For example, the character class expression []−] matches the characters ] and .


A regular expression followed by + means one or more times. For example, [0−9]+ is equivalent to [0−9][0−9]*.

{m} {m,} {m,u}

Integer values enclosed in { } indicate the number of times the preceding regular expression is to be applied. The value m is the minimum number and u is a number, less than 256, which is the maximum. If only m is present (that is, {m}), it indicates the exact number of times the regular expression is to be applied. The value {m,} is analogous to { m,infinity}. The plus (+) and star (*) operations are equivalent to {1,} and {0,} respectively.

( ... )$n

The value of the enclosed regular expression is to be returned. The value will be stored in the (n+1)th argument following the subject argument. At most, ten enclosed regular expressions are allowed. The regex() function makes its assignments unconditionally.

( ... )

Parentheses are used for grouping. An operator, for example, *, +, { }, can work on a single character or a regular expression enclosed in parentheses. For example, (a*(cb+)*)$0. By necessity, all the above defined symbols are special. They must, therefore, be escaped with a \ (backslash) to be used as themselves.


Example 1 Example matching a leading newline in the subject string.

The following example matches a leading newline in the subject string pointed at by cursor.

char *cursor, *newcursor, *ptr;
	. . .
newcursor = regex((ptr = regcmp("^\n", (char *)0)), cursor);

The following example matches through the string Testing3 and returns the address of the character after the last matched character (the “4”). The string Testing3 is copied to the character array ret0.

char ret0[9];
char *newcursor, *name;
	. . .
name = regcmp("([A−Za−z][A−za−z0−9]{0,7})$0", (char *)0);
newcursor = regex(name, "012Testing345", ret0);

The following example applies a precompiled regular expression in file.i (see regcmp(1)) against string.

#include "file.i"
char *string, *newcursor;
	. . .
newcursor = regex(name, string);


See attributes(7) for descriptions of the following attributes:


See Also

ed(1), regcmp(1), malloc(3C), attributes(7), regexp(7)


The user program may run out of memory if regcmp() is called iteratively without freeing the vectors no longer required.

    The regex() and regcmp() functions works only with the ASCII and Oracle Solaris EUC character sets. The EUC character set includes the following encodings:

  • EUC-CN

  • EUC-JP

  • EUC-KR

  • EUC-TW