regcomp(3G) regcomp(3G)
regcomp: regexec, regerror, regfree - regular expression matching
#include <sys/types.h>
#include <regex.h>
int regcomp (regex_t *preg<b>, const char *pattern<b>, int cflags<b>);
int regexec (const regex_t *preg<b>, const char *string<b>, size_t nmatch<b>,
regmatch_t pmatch[], int eflags<b>);
size_t regerror (int errcode<b>, const regex_t *preg<b>, char *errbuf<b>, size_t
errbuf_size<b>);
size_t regfree (regex_t *preg<b>);
The structure type regex_t contains the following members:
MEMBER MEANING
_____________________________________________________________
int re_magic RE magic number
size_t re_nsub number of parenthesized subexpressions
const char *re_endp end pointer for REG_PEND
struct re_guts *re_g internal RE data structure
The structure type regmatch_t contains the following members:
MEMBER MEANING
___________________________________________________________
regoff_t rm_so Byte offset from start of string to start
of substring
regoff_t rm_eo Byte offset from start of string of the
first character after the end of substring
The regcomp() function will compile the regular expression contained in
the string pointed to by the pattern argument and place the results in
the structure pointed to by preg. The cflags argument is the bitwise
inclusive OR of zero or more of the following flags, which are defined in
the header <regex.h>:
_________________________________________________________
REG_EXTENDED Use Extended Regular Expressions.
REG_ICASE Ignore case in match.
REG_NOSUB Report only success/fail in regexec() .
REG_NEWLINE Change the handling of newline characters,
as described in the text.
The default regular expression type for pattern is a Basic Regular
Expression. The application can specify Extended Regular Expressions
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regcomp(3G) regcomp(3G)
using the REG_EXTENDED cflags flag.
On successful completion, it returns 0; otherwise it returns non-zero,
and the content of preg is undefined.
If the REG_NOSUB flag was not set in cflags, then regcomp() will set
re_nsub to the number of parenthesised subexpressions (delimited by \( \)
in basic regular expressions or ( ) in extended regular expressions)
found in pattern.
The regexec() function compares the null-terminated string specified by
string with the compiled regular expression preg initialised by a
previous call to regcomp(). If it finds a match, regexec() returns 0;
otherwise it returns non-zero indicating either no match or an error.
The eflags argument is the bitwise inclusive OR of zero or more of the
following flags, which are defined in the header <regex.h>:
__________________________________________________________
REG_NOTBOL The first character of the string pointed to
by string is not the beginning of the line.
Therefore, the circumflex character (^), when
taken as a special character, will not match
the beginning of string.
REG_NOTEOL The last character of the string pointed to
by string is not the end of the line.
Therefore, the dollar sign ($), when taken
as a special character, will not match the
end of string.
If nmatch is 0 or REG_NOSUB was set in the cflags argument to regcomp() ,
then regexec() will ignore the pmatch argument. Otherwise, the pmatch
argument must point to an array with at least nmatch elements, and
regexec() will fill in the elements of that array with offsets of the
substrings of string that correspond to the parenthesised subexpressions
of pattern: pmatch[i].rm_so will be the byte offset of the beginning and
pmatch[i].rm_eo will be one greater than the byte offset of the end of
substring i. (Subexpression i begins at the ith matched open parenthesis,
counting from 1.) Offsets in pmatch[0] identify the substring that
corresponds to the entire regular expression. Unused elements of pmatch
up to pmatch[nmatch-1] will be filled with -1. If there are more than
nmatch subexpressions in pattern (pattern itself counts as a
subexpression), then regexec() will still do the match, but will record
only the first nmatch substrings.
When matching a basic or extended regular expression, any given
parenthesised subexpression of pattern might participate in the match of
several different substrings of string, or it might not match any
substring even though the pattern as a whole did match. The following
rules are used to determine which substrings to report in pmatch when
matching regular expressions:
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regcomp(3G) regcomp(3G)
1. If subexpression i in a regular expression is not
contained within another subexpression, and it participated in
the match several times, then the byte offsets in pmatch[i]
will delimit the last such match.
2. If subexpression i is not contained within another
subexpression, and it did not participate in an otherwise"
successful match, the byte offsets in pmatch[i] will be -1. A
subexpression does not participate in the match when: * or \{
\} appears immediately after the subexpression in a basic
regular expression, or *, ?, or { } appears immediately after
the subexpression in an extended regular expression, and the
subexpression did not match (matched 0 times)
or:
| is used in an extended regular expression to select this
subexpression or another, and the other subexpression matched.
3.If subexpression i is contained within another
subexpression j, and i is not contained within any other
subexpression that is contained within j, and a match of
subexpression j is reported in pmatch[j], then the match or
non-match of subexpression i reported in pmatch[i] will be as
described in 1. and 2. above, but within the substring reported
in pmatch[j] rather than the whole string.
4.If subexpression i is contained in subexpression j, and
the byte offsets in pmatch[j] are -1, then the pointers in
pmatch[i] also will be -1.
5.If subexpression i matched a zero-length string, then
both byte offsets in pmatch[i] will be the byte offset of the
character or null terminator immediately following the zerolength
string.
If, when regexec() is called, the locale is different from when the
regular expression was compiled, the result is undefined.
If REG_NEWLINE is not set in cflags, then a newline character in pattern
or string will be treated as an ordinary character. If REG_NEWLINE is
set, then newline will be treated as an ordinary character except as
follows:
1.A newline character in string will not be matched by a
period outside a bracket expression or by any form of a nonmatching
list
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regcomp(3G) regcomp(3G)
2.A circumflex (^) in pattern, when used to specify
expression anchoring, will match the zero-length string
immediately after a newline in string, regardless of the
setting of REG_NOTBOL.
3.A dollar-sign ($) in pattern, when used to specify
expression anchoring, will match the zero-length string
immediately before a newline in string, regardless of the
setting of REG_NOTEOL.
The regfree() function frees any memory allocated by regcomp()
associated with preg.
The following constants are defined as error return values:
_____________________________________________________________
REG_NOMATCH regexec() failed to match.
REG_BADPAT Invalid regular expression.
REG_ECOLLATE Invalid collating element referenced.
REG_ECTYPE Invalid character class type referenced.
REG_EESCAPE Trailing \ in pattern.
REG_ESUBREG Number in \digit invalid or in error.
REG_EBRACK [ ] imbalance.
REG_ENOSYS The function is not supported.
REG_EPAREN \( \) or ( ) imbalance.
REG_EBRACE \{ \} imbalance.
REG_BADBR Content of \{ \} invalid: not a number, number
too large, more than two numbers, first
larger than second.
REG_ERANGE Invalid endpoint in range expression.
REG_ESPACE Out of memory.
REG_BADRPT ?, * or + not preceded by valid regular
expression.
The regerror() function provides a mapping from error codes returned by
regcomp() and regexec() to unspecified printable strings. It generates
a string corresponding to the value of the errcode argument, which must
be the last non-zero value returned by regcomp() or regexec() with the
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regcomp(3G) regcomp(3G)
given value of preg. If errcode is not such a value, the content of the
generated string is unspecified.
If preg is a null pointer, but errcode is a value returned by a previous
call to regexec() or regcomp(), the regerror() still generates an error
string corresponding to the value of errcode.
If the errbuf_size argument is not 0, regerror() will place the generated
string into the buffer of size errbuf_size bytes pointed to by errbuf. If
the string (including the terminating null) cannot fit in the buffer,
regerror() will truncate the string and null-terminate the result.
If errbuf_size is 0, regerror() ignores the errbuf argument, and returns
the size of the buffer needed to hold the generated string.
If the preg argument to regexec() or regfree() is not a compiled
regular expression returned by regcomp() , the result is undefined. A
preg is no longer treated as a compiled regular expression after it is
given to regfree() .
On successful completion, the regcomp() function returns 0. Otherwise,
it returns an integer value indicating an error as described in
<regex.h>, and the content of preg is undefined.
On successful completion, the regexec() function returns 0. Otherwise
it returns REG_NOMATCH to indicate no match, or REG_ENOSYS to indicate
that the function is not supported.
Upon successful completion, the regerror() function returns the number
of bytes needed to hold the entire generated string. Otherwise, it
returns 0 to indicate that the function is not implemented.
The regfree() function returns no value.
#include <regex.h>
/*
* Match string against the extended regular expression in
* pattern, treating errors as no match.
*
* return 1 for match, 0 for no match
*/
int match(const char *string, char *pattern) {
int status;
regex_t re;
if (regcomp(&re, pattern, REG_EXTENDED | REG_NOSUB) != 0)
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regcomp(3G) regcomp(3G)
{
return(0); /* report error */
}
status = regexec(&re, string, (size_t) 0, NULL, 0);
regfree(&re);
if (status != 0) {
return(0); /* report error */
}
return(1);
}
The following demonstrates how the REG_NOTBOL flag could be used with
regexec() to find all substrings in a line that match a pattern supplied
by a user. (For simplicity of the example, very little error checking is
done.)
(void) regcomp (&re, pattern, 0);
/* this call to regexec( ) finds the first match
* on the line
*/
error = regexec (&re, &buffer[0], 1, &pm, 0);
while (error == 0) { /* while matches found */
/* substring found between pm.rm_so and pm.rm_eo */
/* This call to regexec( ) finds the next match */
error = regexec (&re, buffer + pm.rm_eo, 1,
&pm, REG_NOTBOL);
}
An application could use:
regerror(code,preg,(char *)NULL,(size_t)0)
to find out how big a buffer is needed for the generated string, malloc()
a buffer to hold the string, and then call regerror() again to get the
string. Alternately, it could allocate a fixed, static buffer that is big
enough to hold most strings, and then use malloc() to allocate a larger
buffer if it finds that this is too small.
fnmatch(3g), glob(3g), <sys/types.h>, <regex.h>
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