.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "REGCOMP" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" regcomp 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
regcomp, regerror, regexec, regfree \- regular expression matching
.SH SYNOPSIS
.LP
\fB#include <regex.h>
.br
.sp
int regcomp(regex_t *restrict\fP \fIpreg\fP\fB, const char *restrict\fP
\fIpattern\fP\fB,
.br
\ \ \ \ \ \  int\fP \fIcflags\fP\fB);
.br
size_t regerror(int\fP \fIerrcode\fP\fB, const regex_t *restrict\fP
\fIpreg\fP\fB,
.br
\ \ \ \ \ \  char *restrict\fP \fIerrbuf\fP\fB, size_t\fP \fIerrbuf_size\fP\fB);
.br
int regexec(const regex_t *restrict\fP \fIpreg\fP\fB, const char *restrict\fP
\fIstring\fP\fB,
.br
\ \ \ \ \ \  size_t\fP \fInmatch\fP\fB, regmatch_t\fP \fIpmatch\fP\fB[restrict],
int\fP
\fIeflags\fP\fB);
.br
void regfree(regex_t *\fP\fIpreg\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
These functions interpret \fIbasic\fP and \fIextended\fP regular expressions
as described in the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Chapter 9, Regular Expressions.
.LP
The \fBregex_t\fP structure is defined in \fI<regex.h>\fP and contains
at least
the following member:
.TS C
center; l2 l2 l.
\fBMember Type\fP	\fBMember Name\fP	\fBDescription\fP
size_t	re_nsub	Number of parenthesized subexpressions.
.TE
.LP
The \fBregmatch_t\fP structure is defined in \fI<regex.h>\fP and contains
at
least the following members:
.TS C
center; l1 l1 lw(40).
\fBMember Type\fP	\fBMember Name\fP	T{
.na
\fBDescription\fP
.ad
T}
\fBregoff_t\fP	\fIrm_so\fP	T{
.na
Byte offset from start of \fIstring\fP to start of substring.
.ad
T}
\fBregoff_t\fP	\fIrm_eo\fP	T{
.na
Byte offset from start of \fIstring\fP of the first character after the end of substring.
.ad
T}
.TE
.LP
The \fIregcomp\fP() function shall compile the regular expression
contained in the string pointed to by the \fIpattern\fP
argument and place the results in the structure pointed to by \fIpreg\fP.
The \fIcflags\fP argument is the bitwise-inclusive OR
of zero or more of the following flags, which are defined in the \fI<regex.h>\fP
header:
.TP 7
REG_EXTENDED
Use Extended Regular Expressions.
.TP 7
REG_ICASE
Ignore case in match. (See the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
Chapter 9, Regular Expressions.)
.TP 7
REG_NOSUB
Report only success/fail in \fIregexec\fP().
.TP 7
REG_NEWLINE
Change the handling of <newline>s, as described in the text.
.sp
.LP
The default regular expression type for \fIpattern\fP is a Basic Regular
Expression. The application can specify Extended
Regular Expressions using the REG_EXTENDED \fIcflags\fP flag.
.LP
If the REG_NOSUB flag was not set in \fIcflags\fP, then \fIregcomp\fP()
shall set \fIre_nsub\fP to the number of
parenthesized subexpressions (delimited by \fB"\\(\\)"\fP in basic
regular expressions or \fB"()"\fP in extended regular
expressions) found in \fIpattern\fP.
.LP
The \fIregexec\fP() function compares the null-terminated string specified
by \fIstring\fP with the compiled regular
expression \fIpreg\fP initialized by a previous call to \fIregcomp\fP().
If it finds a match, \fIregexec\fP() shall return 0;
otherwise, it shall return non-zero indicating either no match or
an error. The \fIeflags\fP argument is the bitwise-inclusive OR
of zero or more of the following flags, which are defined in the \fI<regex.h>\fP
header:
.TP 7
REG_NOTBOL
The first character of the string pointed to by \fIstring\fP is not
the beginning of the line. Therefore, the circumflex
character ( \fB'^'\fP ), when taken as a special character, shall
not match the beginning of \fIstring\fP.
.TP 7
REG_NOTEOL
The last character of the string pointed to by \fIstring\fP is not
the end of the line. Therefore, the dollar sign (
\fB'$'\fP ), when taken as a special character, shall not match the
end of \fIstring\fP.
.sp
.LP
If \fInmatch\fP is 0 or REG_NOSUB was set in the \fIcflags\fP argument
to \fIregcomp\fP(), then \fIregexec\fP() shall ignore
the \fIpmatch\fP argument. Otherwise, the application shall ensure
that the \fIpmatch\fP argument points to an array with at
least \fInmatch\fP elements, and \fIregexec\fP() shall fill in the
elements of that array with offsets of the substrings of
\fIstring\fP that correspond to the parenthesized subexpressions of
\fIpattern\fP: \fIpmatch\fP[ \fIi\fP]. \fIrm_so\fP shall
be the byte offset of the beginning and \fIpmatch\fP[ \fIi\fP]. \fIrm_eo\fP
shall be one greater than the byte offset of the end
of substring \fIi\fP. (Subexpression \fIi\fP begins at the \fIi\fPth
matched open parenthesis, counting from 1.) Offsets in
\fIpmatch\fP[0] identify the substring that corresponds to the entire
regular expression. Unused elements of \fIpmatch\fP up to
\fIpmatch\fP[ \fInmatch\fP-1] shall be filled with -1. If there are
more than \fInmatch\fP subexpressions in \fIpattern\fP (
\fIpattern\fP itself counts as a subexpression), then \fIregexec\fP()
shall still do the match, but shall record only the first
\fInmatch\fP substrings.
.LP
When matching a basic or extended regular expression, any given parenthesized
subexpression of \fIpattern\fP might participate
in the match of several different substrings of \fIstring\fP, or it
might not match any substring even though the pattern as a
whole did match. The following rules shall be used to determine which
substrings to report in \fIpmatch\fP when matching regular
expressions:
.IP " 1." 4
If subexpression \fIi\fP in a regular expression is not contained
within another subexpression, and it participated in the
match several times, then the byte offsets in \fIpmatch\fP[ \fIi\fP]
shall delimit the last such match.
.LP
.IP " 2." 4
If subexpression \fIi\fP is not contained within another subexpression,
and it did not participate in an otherwise successful
match, the byte offsets in \fIpmatch\fP[ \fIi\fP] shall be -1. A subexpression
does not participate in the match when:
\fB'*'\fP or \fB"\\{\\}"\fP appears immediately after the subexpression
in a basic regular expression, or
\fB'*'\fP, \fB'?'\fP, or \fB"{}"\fP appears immediately after the
subexpression in an extended regular expression, and
the subexpression did not match (matched 0 times)
.LP
or:
\fB'|'\fP is used in an extended regular expression to select this
subexpression or another, and the other
subexpression matched.
.LP
.IP " 3." 4
If subexpression \fIi\fP is contained within another subexpression
\fIj\fP, and \fIi\fP is not contained within any other
subexpression that is contained within \fIj\fP, and a match of subexpression
\fIj\fP is reported in \fIpmatch\fP[ \fIj\fP],
then the match or non-match of subexpression \fIi\fP reported in \fIpmatch\fP[
\fIi\fP] shall be as described in 1. and 2.
above, but within the substring reported in \fIpmatch\fP[ \fIj\fP]
rather than the whole string. The offsets in \fIpmatch\fP[
\fIi\fP] are still relative to the start of \fIstring\fP.
.LP
.IP " 4." 4
If subexpression \fIi\fP is contained in subexpression \fIj\fP, and
the byte offsets in \fIpmatch\fP[ \fIj\fP] are -1, then
the pointers in \fIpmatch\fP[ \fIi\fP] shall also be -1.
.LP
.IP " 5." 4
If subexpression \fIi\fP matched a zero-length string, then both byte
offsets in \fIpmatch\fP[ \fIi\fP] shall be the byte
offset of the character or null terminator immediately following the
zero-length string.
.LP
.LP
If, when \fIregexec\fP() is called, the locale is different from when
the regular expression was compiled, the result is
undefined.
.LP
If REG_NEWLINE is not set in \fIcflags\fP, then a <newline> in \fIpattern\fP
or \fIstring\fP shall be treated as an
ordinary character. If REG_NEWLINE is set, then <newline> shall be
treated as an ordinary character except as follows:
.IP " 1." 4
A <newline> in \fIstring\fP shall not be matched by a period outside
a bracket expression or by any form of a
non-matching list (see the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
Chapter
9, Regular Expressions).
.LP
.IP " 2." 4
A circumflex ( \fB'^'\fP ) in \fIpattern\fP, when used to specify
expression anchoring (see the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Section 9.3.8, BRE Expression Anchoring),
shall match the zero-length string immediately after a <newline> in
\fIstring\fP, regardless of the setting of
REG_NOTBOL.
.LP
.IP " 3." 4
A dollar sign ( \fB'$'\fP ) in \fIpattern\fP, when used to specify
expression anchoring, shall match the zero-length string
immediately before a <newline> in \fIstring\fP, regardless of the
setting of REG_NOTEOL.
.LP
.LP
The \fIregfree\fP() function frees any memory allocated by \fIregcomp\fP()
associated with \fIpreg\fP.
.LP
The following constants are defined as error return values:
.TP 7
REG_NOMATCH
\fIregexec\fP() failed to match.
.TP 7
REG_BADPAT
Invalid regular expression.
.TP 7
REG_ECOLLATE
Invalid collating element referenced.
.TP 7
REG_ECTYPE
Invalid character class type referenced.
.TP 7
REG_EESCAPE
Trailing \fB'\\'\fP in pattern.
.TP 7
REG_ESUBREG
Number in \fB"\\digit"\fP invalid or in error.
.TP 7
REG_EBRACK
\fB"[]"\fP imbalance.
.TP 7
REG_EPAREN
\fB"\\(\\)"\fP or \fB"()"\fP imbalance.
.TP 7
REG_EBRACE
\fB"\\{\\}"\fP imbalance.
.TP 7
REG_BADBR
Content of \fB"\\{\\}"\fP invalid: not a number, number too large,
more than two numbers, first larger than second.
.TP 7
REG_ERANGE
Invalid endpoint in range expression.
.TP 7
REG_ESPACE
Out of memory.
.TP 7
REG_BADRPT
\fB'?'\fP, \fB'*'\fP, or \fB'+'\fP not preceded by valid regular
expression.
.sp
.LP
The \fIregerror\fP() function provides a mapping from error codes
returned by \fIregcomp\fP() and \fIregexec\fP() to
unspecified printable strings. It generates a string corresponding
to the value of the \fIerrcode\fP argument, which the
application shall ensure is the last non-zero value returned by \fIregcomp\fP()
or \fIregexec\fP() with the given value of
\fIpreg\fP. If \fIerrcode\fP is not such a value, the content of the
generated string is unspecified.
.LP
If \fIpreg\fP is a null pointer, but \fIerrcode\fP is a value returned
by a previous call to \fIregexec\fP() or
\fIregcomp\fP(), the \fIregerror\fP() still generates an error string
corresponding to the value of \fIerrcode\fP, but it might
not be as detailed under some implementations.
.LP
If the \fIerrbuf_size\fP argument is not 0, \fIregerror\fP() shall
place the generated string into the buffer of size
\fIerrbuf_size\fP bytes pointed to by \fIerrbuf\fP. If the string
(including the terminating null) cannot fit in the buffer,
\fIregerror\fP() shall truncate the string and null-terminate the
result.
.LP
If \fIerrbuf_size\fP is 0, \fIregerror\fP() shall ignore the \fIerrbuf\fP
argument, and return the size of the buffer needed
to hold the generated string.
.LP
If the \fIpreg\fP argument to \fIregexec\fP() or \fIregfree\fP() is
not a compiled regular expression returned by
\fIregcomp\fP(), the result is undefined. A \fIpreg\fP is no longer
treated as a compiled regular expression after it is given to
\fIregfree\fP().
.SH RETURN VALUE
.LP
Upon successful completion, the \fIregcomp\fP() function shall return
0. Otherwise, it shall return an integer value indicating
an error as described in \fI<regex.h>\fP, and the content of \fIpreg\fP
is
undefined. If a code is returned, the interpretation shall be as given
in \fI<regex.h>\fP.
.LP
If \fIregcomp\fP() detects an invalid RE, it may return REG_BADPAT,
or it may return one of the error codes that more precisely
describes the error.
.LP
Upon successful completion, the \fIregexec\fP() function shall return
0. Otherwise, it shall return REG_NOMATCH to indicate no
match.
.LP
Upon successful completion, the \fIregerror\fP() function shall return
the number of bytes needed to hold the entire generated
string, including the null termination. If the return value is greater
than \fIerrbuf_size\fP, the string returned in the buffer
pointed to by \fIerrbuf\fP has been truncated.
.LP
The \fIregfree\fP() function shall not return a value.
.SH ERRORS
.LP
No errors are defined.
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.sp
.RS
.nf

\fB#include <regex.h>
.sp

/*
 * Match string against the extended regular expression in
 * pattern, treating errors as no match.
 *
 * Return 1 for match, 0 for no match.
 */
.sp

int
match(const char *string, char *pattern)
{
    int    status;
    regex_t    re;
.sp

    if (regcomp(&re, pattern, REG_EXTENDED|REG_NOSUB) != 0) {
        return(0);      /* Report error. */
    }
    status = regexec(&re, string, (size_t) 0, NULL, 0);
    regfree(&re);
    if (status != 0) {
        return(0);      /* Report error. */
    }
    return(1);
}
\fP
.fi
.RE
.LP
The following demonstrates how the REG_NOTBOL flag could be used with
\fIregexec\fP() 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.)
.sp
.RS
.nf

\fB(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);
}
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
An application could use:
.sp
.RS
.nf

\fBregerror(code,preg,(char *)NULL,(size_t)0)
\fP
.fi
.RE
.LP
to find out how big a buffer is needed for the generated string, \fImalloc\fP()
a buffer
to hold the string, and then call \fIregerror\fP() again to get the
string. Alternatively, it could allocate a fixed, static
buffer that is big enough to hold most strings, and then use \fImalloc\fP()
to allocate a
larger buffer if it finds that this is too small.
.LP
To match a pattern as described in the Shell and Utilities volume
of IEEE\ Std\ 1003.1-2001, Section 2.13, Pattern Matching Notation,
use the \fIfnmatch\fP() function.
.SH RATIONALE
.LP
The \fIregexec\fP() function must fill in all \fInmatch\fP elements
of \fIpmatch\fP, where \fInmatch\fP and \fIpmatch\fP
are supplied by the application, even if some elements of \fIpmatch\fP
do not correspond to subexpressions in \fIpattern\fP. The
application writer should note that there is probably no reason for
using a value of \fInmatch\fP that is larger than
\fIpreg\fP-> \fIre_nsub\fP+1.
.LP
The REG_NEWLINE flag supports a use of RE matching that is needed
in some applications like text editors. In such applications,
the user supplies an RE asking the application to find a line that
matches the given expression. An anchor in such an RE anchors at
the beginning or end of any line. Such an application can pass a sequence
of <newline>-separated lines to \fIregexec\fP() as
a single long string and specify REG_NEWLINE to \fIregcomp\fP() to
get the desired behavior. The application must ensure that
there are no explicit <newline>s in \fIpattern\fP if it wants to ensure
that any match occurs entirely within a single
line.
.LP
The REG_NEWLINE flag affects the behavior of \fIregexec\fP(), but
it is in the \fIcflags\fP parameter to \fIregcomp\fP() to
allow flexibility of implementation. Some implementations will want
to generate the same compiled RE in \fIregcomp\fP() regardless
of the setting of REG_NEWLINE and have \fIregexec\fP() handle anchors
differently based on the setting of the flag. Other
implementations will generate different compiled REs based on the
REG_NEWLINE.
.LP
The REG_ICASE flag supports the operations taken by the \fIgrep\fP
\fB-i\fP option and
the historical implementations of \fIex\fP and \fIvi\fP.
Including this flag will make it easier for application code to be
written that does the same thing as these utilities.
.LP
The substrings reported in \fIpmatch\fP[] are defined using offsets
from the start of the string rather than pointers. Since
this is a new interface, there should be no impact on historical implementations
or applications, and offsets should be just as
easy to use as pointers. The change to offsets was made to facilitate
future extensions in which the string to be searched is
presented to \fIregexec\fP() in blocks, allowing a string to be searched
that is not all in memory at once.
.LP
The type \fBregoff_t\fP is used for the elements of \fIpmatch\fP[]
to ensure that the application can represent either the
largest possible array in memory (important for an application conforming
to the Shell and Utilities volume of
IEEE\ Std\ 1003.1-2001) or the largest possible file (important for
an application using the extension where a file is
searched in chunks).
.LP
The standard developers rejected the inclusion of a \fIregsub\fP()
function that would be used to do substitutions for a
matched RE. While such a routine would be useful to some applications,
its utility would be much more limited than the matching
function described here. Both RE parsing and substitution are possible
to implement without support other than that required by the
ISO\ C standard, but matching is much more complex than substituting.
The only difficult part of substitution, given the
information supplied by \fIregexec\fP(), is finding the next character
in a string when there can be multi-byte characters. That
is a much larger issue, and one that needs a more general solution.
.LP
The \fIerrno\fP variable has not been used for error returns to avoid
filling the \fIerrno\fP name space for this feature.
.LP
The interface is defined so that the matched substrings \fIrm_sp\fP
and \fIrm_ep\fP are in a separate \fBregmatch_t\fP
structure instead of in \fBregex_t\fP. This allows a single compiled
RE to be used simultaneously in several contexts; in
\fImain\fP() and a signal handler, perhaps, or in multiple threads
of lightweight processes. (The \fIpreg\fP argument to
\fIregexec\fP() is declared with type \fBconst\fP, so the implementation
is not permitted to use the structure to store
intermediate results.) It also allows an application to request an
arbitrary number of substrings from an RE. The number of
subexpressions in the RE is reported in \fIre_nsub\fP in \fIpreg\fP.
With this change to \fIregexec\fP(), consideration was
given to dropping the REG_NOSUB flag since the user can now specify
this with a zero \fInmatch\fP argument to \fIregexec\fP().
However, keeping REG_NOSUB allows an implementation to use a different
(perhaps more efficient) algorithm if it knows in
\fIregcomp\fP() that no subexpressions need be reported. The implementation
is only required to fill in \fIpmatch\fP if
\fInmatch\fP is not zero and if REG_NOSUB is not specified. Note that
the \fBsize_t\fP type, as defined in the ISO\ C
standard, is unsigned, so the description of \fIregexec\fP() does
not need to address negative values of \fInmatch\fP.
.LP
REG_NOTBOL was added to allow an application to do repeated searches
for the same pattern in a line. If the pattern contains a
circumflex character that should match the beginning of a line, then
the pattern should only match when matched against the
beginning of the line. Without the REG_NOTBOL flag, the application
could rewrite the expression for subsequent matches, but in the
general case this would require parsing the expression. The need for
REG_NOTEOL is not as clear; it was added for symmetry.
.LP
The addition of the \fIregerror\fP() function addresses the historical
need for conforming application programs to have access
to error information more than "Function failed to compile/match your
RE for unknown reasons".
.LP
This interface provides for two different methods of dealing with
error conditions. The specific error codes (REG_EBRACE, for
example), defined in \fI<regex.h>\fP, allow an application to recover
from an error
if it is so able. Many applications, especially those that use patterns
supplied by a user, will not try to deal with specific
error cases, but will just use \fIregerror\fP() to obtain a human-readable
error message to present to the user.
.LP
The \fIregerror\fP() function uses a scheme similar to \fIconfstr\fP()
to deal with
the problem of allocating memory to hold the generated string. The
scheme used by \fIstrerror\fP() in the ISO\ C standard was considered
unacceptable since it creates
difficulties for multi-threaded applications.
.LP
The \fIpreg\fP argument is provided to \fIregerror\fP() to allow an
implementation to generate a more descriptive message than
would be possible with \fIerrcode\fP alone. An implementation might,
for example, save the character offset of the offending
character of the pattern in a field of \fIpreg\fP, and then include
that in the generated message string. The implementation may
also ignore \fIpreg\fP.
.LP
A REG_FILENAME flag was considered, but omitted. This flag caused
\fIregexec\fP() to match patterns as described in the Shell
and Utilities volume of IEEE\ Std\ 1003.1-2001, Section 2.13, Pattern
Matching Notation instead of REs. This service is now provided by
the \fIfnmatch\fP()
function.
.LP
Notice that there is a difference in philosophy between the ISO\ POSIX-2:1993
standard and IEEE\ Std\ 1003.1-2001 in
how to handle a "bad" regular expression. The ISO\ POSIX-2:1993 standard
says that many bad constructs "produce undefined
results", or that "the interpretation is undefined". IEEE\ Std\ 1003.1-2001,
however, says that the interpretation of
such REs is unspecified. The term "undefined" means that the action
by the application is an error, of similar severity to
passing a bad pointer to a function.
.LP
The \fIregcomp\fP() and \fIregexec\fP() functions are required to
accept any null-terminated string as the \fIpattern\fP
argument. If the meaning of the string is "undefined", the behavior
of the function is "unspecified".
IEEE\ Std\ 1003.1-2001 does not specify how the functions will interpret
the pattern; they might return error codes, or
they might do pattern matching in some completely unexpected way,
but they should not do something like abort the process.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIfnmatch\fP(), \fIglob\fP(), Shell and Utilities volume of
IEEE\ Std\ 1003.1-2001, Section 2.13, Pattern Matching Notation, Base
Definitions volume of IEEE\ Std\ 1003.1-2001, Chapter 9, Regular Expressions,
\fI<regex.h>\fP, \fI<sys/types.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .