.\" Copyright (c) 2011 Gabor Kovesdan <gabor@FreeBSD.org>.
.\" Copyright (c) 1992, 1993, 1994 Henry Spencer.
.\" Copyright (c) 1992, 1993, 1994
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Henry Spencer.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
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.\"	@(#)regex.3	8.4 (Berkeley) 3/20/94
.\" $FreeBSD$
.\"
.Dd December 23, 2011
.Dt REGEX 3
.Os
.Sh NAME
.Nm regcomp ,
.Nm regncomp ,
.Nm regwcomp ,
.Nm regwncomp ,
.Nm regexec ,
.Nm regnexec ,
.Nm regwexec ,
.Nm regwnexec ,
.Nm regerror ,
.Nm regfree
.Nd regular-expression library
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In regex.h
.Ft int
.Fo regcomp
.Fa "regex_t * preg" "const char * pattern" "int cflags"
.Fc
.Ft int
.Fo regncomp
.Fa "regex_t * preg" "const char * pattern" "size_t len" "int cflags"
.Fc
.Ft int
.Fo regwcomp
.Fa "regex_t * preg" "const wchar_t * pattern" "int cflags"
.Fc
.Ft int
.Fo regwncomp
.Fa "regex_t * preg" "const wchar_t * pattern" "size_t len" "int cflags"
.Fc
.Ft int
.Fo regexec
.Fa "const regex_t * preg" "const char * string"
.Fa "size_t nmatch" "regmatch_t pmatch[]" "int eflags"
.Fc
.Ft int
.Fo regnexec
.Fa "const regex_t * preg" "const char * string" "size_t len"
.Fa "size_t nmatch" "regmatch_t pmatch[]" "int eflags"
.Fc
.Ft int
.Fo regwexec
.Fa "const regex_t * preg" "const wchar_t * string"
.Fa "size_t nmatch" "regmatch_t pmatch[]" "int eflags"
.Fc
.Ft int
.Fo regwnexec
.Fa "const regex_t * preg" "const wchar_t * string" "size_t len"
.Fa "size_t nmatch" "regmatch_t pmatch[]" "int eflags"
.Fc
.Ft size_t
.Fo regerror
.Fa "int errcode" "const regex_t * restrict preg"
.Fa "char * restrict errbuf" "size_t errbuf_size"
.Fc
.Ft void
.Fn regfree "regex_t *preg"
.Sh DESCRIPTION
These routines implement pattern matching of
.St -p1003.2
regular expressions.
The
.Xr re_format 7
manual can be consulted for their syntax and usage.
.Pp
The
.Fn regcomp
function
compiles a regular expression from a string into an internal form.
The
.Fn regncomp
function works like the former but takes another argument to
specify the length of the pattern.
This function can accept patterns containing NUL bytes.
The
.Fn regwcomp
and
.Fn regwncomp
functions work like the two former ones but take the pattern in
the wide string form.
.Pp
The
.Fn regexec
function matches the compiled regular expression against a string
and reports results.
The
.Fn regnexec
function works like the former but takes another argument to specify
the length of the pattern,
allowing NUL bytes in the input string.
Additionally,
for long inputs strings it is more efficient to call this function if
the length is already known beause it will not require the matcher to
calculate the length and read the input bytes one by one.
The
.Fn regwexec
and
.Fn regwnexec
functions work like the two former ones but take the input as a
wide string.
.Pp
The
.Fn regerror
function transforms error codes from the above functions into
human-readable messages.
.Pp
The
.Fn regfree
function frees any dynamically-allocated storage used by the internal form
of a regular expression.
.Pp
The header
.In regex.h
declares two structure types,
.Ft regex_t
and
.Ft regmatch_t ,
the former for compiled internal forms and the latter for submatch reporting.
It also declares the functions mentioned above,
a type
.Ft regoff_t ,
and a number of constants with names starting with
.Dq Dv REG_ .
.Pp
The
.Fn regcomp
family of functions compile the regular expression contained in the
.Fa pattern
string,
subject to the flags in
.Fa cflags ,
and places the results in the
.Ft regex_t
structure pointed to by
.Fa preg .
Some variants of the function also take the length of the pattern in
.Fa len.
The
.Fa cflags
argument
is the bitwise OR of zero or more of the following flags:
.Bl -tag -width REG_EXTENDED
.It Dv REG_EXTENDED
Compile extended regular expressions
.Pq Dq EREs ,
rather than the obsolete basic regular expressions
.Pq Dq BREs
that are the default.
It may not be used together with
.Dv REG_NOSPEC
or
.Dv REG_LITERAL
in the same call to
.Fn regcomp .
.It Dv REG_BASIC
This is a synonym for 0,
provided as a counterpart to
.Dv REG_EXTENDED
to improve readability.
.It Dv REG_NOSPEC
Compile with recognition of all special characters turned off.
All characters are thus considered ordinary,
so the reqular expression is a literal string.
.It Dv REG_LITERAL
Synonym for
.Dv REG_NOSPEC.
.It Dv REG_ICASE
Compile for case insensitive matching.
.It Dv REG_NOSUB
Compile for only reporting match or mismatch with no regard to
the matching offset.
.It Dv REG_NEWLINE
Compile for newline-sensitive matching.
By default, newline is a completely ordinary character with no special
meaning in either regular expressions or strings.
With this flag,
.Ql [^
bracket expressions and
.Ql .\&
never match newline,
a
.Ql ^\&
anchor matches the null string after any newline in the string
in addition to its normal function,
and the
.Ql $\&
anchor matches the null string before any newline in the
string in addition to its normal function.
.It Dv REG_PEND
The regular expression ends,
not at the first NUL,
but just before the character pointed to by the
.Va re_endp
or
.Va re_wendp
member of the structure pointed to by
.Fa preg .
The former is used for the functions that take a single- or multi-byte
string,
while the second is used for those taking a wide string.
The
.Va re_endp
member is of type
.Ft "const char *" 
and the
.Va re_wendp
member is of type
.Ft "const wchar_t *" .
This flag permits inclusion of NULs in the regular expression;
they are considered ordinary characters.
.El
.Pp
When successful,
the
.Fn regcomp
family of functions returns
.Dv REG_OK
and fills in the structure pointed to by
.Fa preg .
The
.Va re_nsub ,
member of the structure of type
.Ft size_t ,
contains the number of parenthesized subexpressions within the regular
expression (except when the
.Dv REG_NOSUB
flag was used for the compilation of the pattern).
If
.Fn regcomp
fails, it returns a non-zero error code;
see
.Sx RETURN VALUES .
.Pp
The
.Fn regexec
family of functions match the compiled regular expression pointed to by
.Fa preg
against the
.Fa string
(possibly having a length of
.Fa len
when using the variants that take the input length),
subject to the flags in
.Fa eflags
and reports through its return value whether the input matches.
The
.Fa pmatch
argument is also filled in to hold submatches unless the pattern was
compiled using the
.Dv REG_NOSUB
flag or the
.Fa nmatch
argument was set to 0.
The regular expression must have been compiled by a previous invocation of
.Fn regcomp
or any of its alternative forms.
The compiled form is not altered during execution of
.Fn regexec
or its alternatives,
so a single compiled regular expression can be used simultaneously by
multiple threads,
and it can be used with any variant of the
.Fn regexec
functions.
(That is,
a multi-byte pattern can be matched to wide string input and
vice versa.)
.Pp
The
.Fa eflags
argument is the bitwise OR of zero or more of the following flags:
.Bl -tag -width REG_STARTEND
.It Dv REG_NOTBOL
The first character of
the string
is not the beginning of a line, so the
.Ql ^\&
anchor should not match before it.
This does not affect the behavior of newlines under
.Dv REG_NEWLINE .
.It Dv REG_NOTEOL
The NUL character terminating
the string
does not end a line, so the
.Ql $\&
anchor should not match before it.
This does not affect the behavior of newlines under
.Dv REG_NEWLINE .
.It Dv REG_STARTEND
The string is considered to start at
.Fa string
+
.Fa pmatch Ns [0]. Ns Va rm_so
(inclusive) and to end at
.Fa string
+
.Fa pmatch Ns [0]. Ns Va rm_eo
(exclusive),
regardless of the value of
.Fa nmatch .
See below for the definition of
.Fa pmatch
and
.Fa nmatch .
Note that a non-zero
.Va rm_so
does not imply
.Dv REG_NOTBOL ;
.Dv REG_STARTEND
affects only the location of the string,
not how it is matched.
.El
.Pp
The function indicates a match by returning
.Dv REG_OK ,
no match with
.Dv REG_NOMATCH ,
or returns an error code different from the above two values
if an error has occured during the execution.
See
.Sx RETURN VALUES
for a detailed description of error codes.
.Pp
If
.Dv REG_NOSUB
was specified in the compilation of the regular expression,
or if
.Fa nmatch
is 0,
.Fn regexec
ignores the
.Fa pmatch
argument (but see below for the case where
.Dv REG_STARTEND
is specified).
Otherwise,
.Fa pmatch
points to an array of
.Fa nmatch
structures of type
.Ft regmatch_t .
Such a structure has at least the members
.Va rm_so
and
.Va rm_eo ,
both of type
.Ft regoff_t
(a signed arithmetic type at least as large as an
.Ft off_t
and a
.Ft 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
.Fa string
argument given to
.Fn regexec .
An empty substring is denoted by equal offsets,
both indicating the character following the empty substring.
.Pp
The 0th member of the
.Fa pmatch
array is filled in to indicate what substring of
.Fa string
was matched by the entire regular expression.
Remaining members report what substring was matched by parenthesized
subexpressions within the regular expression;
member
.Va i
reports subexpression
.Va i ,
with subexpressions counted (starting at 1) by the order of their opening
parentheses in the regular expression, 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 regular expression (that is,
.Va i
>
.Fa preg Ns -> Ns Va re_nsub ) )
have both
.Va rm_so
and
.Va rm_eo
set to -1.
If a subexpression participated in the match several times,
the reported substring is the last one it matched.
(Note, as an example in particular, that when the regular expression
.Ql "(b*)+"
matches
.Ql bbb ,
the parenthesized subexpression matches each of the three
.So Li b Sc Ns s
and then
an infinite number of empty strings following the last
.Ql b ,
so the reported substring is one of the empties.)
.Pp
If
.Dv REG_STARTEND
is specified,
.Fa pmatch
must point to at least one
.Ft regmatch_t
(even if
.Fa nmatch
is 0 or
.Dv REG_NOSUB
was specified),
to hold the input offsets for
.Dv REG_STARTEND .
Use for output is still entirely controlled by
.Fa nmatch ;
if
.Fa nmatch
is 0 or
.Dv REG_NOSUB
was specified,
the value of
.Fa pmatch Ns [0]
will not be changed by a successful
.Fn regexec .
.Pp
The
.Fn regerror
function
maps a non-zero
.Fa errcode
from either
.Fn regcomp
or
.Fn regexec
to a human-readable, printable message.
If
.Fa preg
is
.No non\- Ns Dv NULL ,
the error code should have arisen from use of
the
.Ft regex_t
pointed to by
.Fa preg ,
and if the error code came from
.Fn regcomp ,
it should have been the result from the most recent
.Fn regcomp
using that
.Ft regex_t .
(The
.Fn regerror
function may be able to supply a more detailed message using information
from the
.Ft regex_t .
)
The
.Fn regerror
function
places the NUL-terminated message into the buffer pointed to by
.Fa errbuf ,
limiting the length (including the NUL) to at most
.Fa errbuf_size
bytes.
If the whole message will not fit,
as much of it as will fit before the terminating NUL is supplied.
In any case,
the returned value is the size of buffer needed to hold the whole
message (including terminating NUL).
If
.Fa errbuf_size
is 0,
.Fa errbuf
is ignored but the return value is still correct.
.Pp
The
.Fn regfree
function
frees any dynamically-allocated storage associated with the compiled
regular expression pointed to by
.Fa preg .
The remaining
.Ft regex_t
is no longer a valid compiled regular expression
and the effect of supplying it to
.Fn regexec
or
.Fn regerror
is undefined.
.Pp
All of the functions described above are thread-safe.
.Sh RETURN VALUES
Non-zero error codes from the
.Fn regcomp
and
.Fn regexec
family of functions
include the following:
.Pp
.Bl -tag -width REG_ECOLLATE -compact
.It Dv REG_OK
Operation successfully executed.
Synonym for 0,
to provide better code readability.
.It Dv REG_NOMATCH
The
.Fn regexec
function or its variants
failed to match.
.It Dv REG_BADPAT
Invalid regular expression.
This implementation only returns this code when the regular expression
passed to
.Fn regcomp
contains an illegal multibyte sequence.
.It Dv REG_ECOLLATE
Invalid collating element.
Returned whenever equivalence classes or multicharacter collating elements
are used in a bracket expression.
.Pq They are not supported yet.
.It Dv REG_ECTYPE
Invalid character class name.
.It Dv REG_EESCAPE
The last character was a backslash.
.It Dv REG_ESUBREG
Invalid backreference number.
.It Dv REG_EBRACK
Brackets
.Ql "[ ]"
are not balanced.
.It Dv REG_EPAREN
Parentheses
.Ql "( )"
are not balanced.
.It Dv REG_EBRACE
Braces
.Ql "{ }"
are not balanced.
.It Dv REG_BADBR
Invalid repetition count(s) in
.Ql "{ }" :
not a number, more than two numbers, first larger than second, or number too large.
.It Dv REG_ERANGE
Invalid character range in
.Ql "[ ]" ,
i.e. ending point is earlier in the collating order than the starting point.
.It Dv REG_ESPACE
Out of memory.
.It Dv REG_BADRPT
Invalid use of repetition operators: two or more repetition operators have been
chained in an undefined way.
.Sh SEE ALSO
.Xr grep 1 ,
.Xr re_format 7
.Pp
.St -p1003.2 ,
sections 2.8 (Regular Expression Notation)
and
B.5 (C Binding for Regular Expression Matching).
.Sh STANDARDS
The
.Fn regcomp ,
.Fn regexec ,
.Fn regerror
and
.Fn regfree
functions,
the header file
.In regex.h
and the two structure types
.Ft regex_t
and
.Ft regmatch_t
(except the
.Va re_endp
and
.Va re_wendp
fields),
the type
.Ft regoff_t ,
the macros
.Dv REG_EXTENDED ,
.Dv REG_ICASE ,
.Dv REG_NOSUB ,
.Dv REG_NEWLINE ,
.Dv REG_NOTBOL ,
.Dv REG_NOTEOL
and all the error codes except
.Dv REG_OK
conform to the standard
.St -p1003.2 .
.Pp
The alternative forms of the functions taking the length of the input and/or
taking wide strings, the flags that are not listed above, the
.Va re_end
and
.Va re_wendp
fields in
.Ft regex_t 
and the
.Dv REG_OK error code  are extensions and thus are not expected to be
portable.
.Sh HISTORY
This regex implementation comes from the TRE project
and it was included first in
.Fx 10-CURRENT.
This manual was originally written by
.An Henry Spencer
for an older implementation and later extended and
tailored for TRE by
.An Gabor Kovesdan .