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REGEX(3) REGEX(3)
NNAAMMEE
regcomp, regexec, regerror, regfree - regular-expression
library
SSYYNNOOPPSSIISS
##iinncclluuddee <<ssyyss//ttyyppeess..hh>>
##iinncclluuddee <<rreeggeexx..hh>>
int regcomp(regex_t *preg, const char *pattern,
int cflags);
int regexec(const regex_t *preg, const char *string,
size_t nmatch, regmatch_t pmatch[], int eflags);
size_t regerror(int errcode, const regex_t *preg,
char *errbuf, size_t errbuf_size);
void regfree(regex_t *preg);
DDEESSCCRRIIPPTTIIOONN
These routines implement POSIX 1003.2 regular expressions
(``RE''s); see _r_e___f_o_r_m_a_t(7)_. _R_e_g_c_o_m_p compiles an RE writ-
ten as a string into an internal form, _r_e_g_e_x_e_c matches
that internal form against a string and reports results,
_r_e_g_e_r_r_o_r transforms error codes from either into human-
readable messages, and _r_e_g_f_r_e_e frees any dynamically-
allocated storage used by the internal form of an RE.
The header _<_r_e_g_e_x_._h_> declares two structure types, _r_e_g_e_x___t
and _r_e_g_m_a_t_c_h___t, the former for compiled internal forms and
the latter for match reporting. It also declares the four
functions, a type _r_e_g_o_f_f___t, and a number of constants with
names starting with ``REG_''.
_R_e_g_c_o_m_p compiles the regular expression contained in the
_p_a_t_t_e_r_n string, subject to the flags in _c_f_l_a_g_s, and places
the results in the _r_e_g_e_x___t structure pointed to by _p_r_e_g.
_C_f_l_a_g_s is the bitwise OR of zero 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 coun-
terpart to REG_EXTENDED to improve readabil-
ity.
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, com-
patible with but not specified by POSIX
1003.2, and should be used with caution in
March 20, 1994 1
REGEX(3) REGEX(3)
software intended to be portable to other
systems. REG_EXTENDED and REG_NOSPEC may
not be used in the same call to _r_e_g_c_o_m_p.
REG_ICASE Compile for matching that ignores
upper/lower case distinctions. See
_r_e___f_o_r_m_a_t(7)_.
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 new-
line, a `^' anchor matches the null string
after any newline in the string in addition
to its normal function, and the `$' anchor
matches the null string before any newline
in the string in addition to its normal
function.
REG_PEND The regular expression ends, not at the
first NUL, but just before the character
pointed to by the _r_e___e_n_d_p member of the
structure pointed to by _p_r_e_g. The _r_e___e_n_d_p
member is of type _c_o_n_s_t _c_h_a_r _*. This flag
permits inclusion of NULs in the RE; they
are considered ordinary characters. This is
an extension, compatible with but not speci-
fied by POSIX 1003.2, and should be used
with caution in software intended to be
portable to other systems.
When successful, _r_e_g_c_o_m_p returns 0 and fills in the struc-
ture pointed to by _p_r_e_g. One member of that structure
(other than _r_e___e_n_d_p) is publicized: _r_e___n_s_u_b, of type
_s_i_z_e___t, contains the number of parenthesized subexpres-
sions within the RE (except that the value of this member
is undefined if the REG_NOSUB flag was used). If _r_e_g_c_o_m_p
fails, it returns a non-zero error code; see DIAGNOSTICS.
_R_e_g_e_x_e_c matches the compiled RE pointed to by _p_r_e_g against
the _s_t_r_i_n_g, subject to the flags in _e_f_l_a_g_s, and reports
results using _n_m_a_t_c_h, _p_m_a_t_c_h, and the returned value. The
RE must have been compiled by a previous invocation of
_r_e_g_c_o_m_p. The compiled form is not altered during execu-
tion of _r_e_g_e_x_e_c, so a single compiled RE can be used
simultaneously by multiple threads.
By default, the NUL-terminated string pointed to by _s_t_r_i_n_g
is considered to be the text of an entire line, minus any
terminating newline. The _e_f_l_a_g_s argument is the bitwise
March 20, 1994 2
REGEX(3) REGEX(3)
OR of zero or more of the following flags:
REG_NOTBOL The first character of the string is not the
beginning of a line, so the `^' anchor
should not match before it. This does not
affect the behavior of newlines under
REG_NEWLINE.
REG_NOTEOL The NUL terminating the string does not end
a line, so the `$' anchor should not match
before it. This does not affect the behav-
ior of newlines under REG_NEWLINE.
REG_STARTEND The string is considered to start at
_s_t_r_i_n_g + _p_m_a_t_c_h[0]._r_m___s_o and to have a ter-
minating NUL located at _s_t_r_i_n_g +
_p_m_a_t_c_h[0]._r_m___e_o (there need not actually be
a NUL at that location), regardless of the
value of _n_m_a_t_c_h. See below for the defini-
tion of _p_m_a_t_c_h and _n_m_a_t_c_h. 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
_r_m___s_o does not imply REG_NOTBOL;
REG_STARTEND affects only the location of
the string, not how it is matched.
See _r_e___f_o_r_m_a_t(7) for a discussion of what is matched in
situations where an RE or a portion thereof could match
any of several substrings of _s_t_r_i_n_g.
Normally, _r_e_g_e_x_e_c 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 DIAGNOS-
TICS.
If REG_NOSUB was specified in the compilation of the RE,
or if _n_m_a_t_c_h is 0, _r_e_g_e_x_e_c ignores the _p_m_a_t_c_h argument
(but see below for the case where REG_STARTEND is speci-
fied). Otherwise, _p_m_a_t_c_h points to an array of _n_m_a_t_c_h
structures of type _r_e_g_m_a_t_c_h___t. Such a structure has at
least the members _r_m___s_o and _r_m___e_o, both of type _r_e_g_o_f_f___t
(a signed arithmetic type at least as large as an _o_f_f___t
and a _s_s_i_z_e___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 _s_t_r_i_n_g argument given
to _r_e_g_e_x_e_c. An empty substring is denoted by equal off-
sets, both indicating the character following the empty
substring.
The 0th member of the _p_m_a_t_c_h array is filled in to indi-
cate what substring of _s_t_r_i_n_g was matched by the entire
March 20, 1994 3
REGEX(3) REGEX(3)
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 par-
ticipate in the match at all, or to subexpressions that do
not exist in the RE (that is, _i > _p_r_e_g->_r_e___n_s_u_b)--have
both _r_m___s_o and _r_m___e_o set to -1. If a subexpression par-
ticipated in the match several times, the reported sub-
string is the last one it matched. (Note, as an example
in particular, 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', so the reported substring is one of the emp-
ties.)
If REG_STARTEND is specified, _p_m_a_t_c_h must point to at
least one _r_e_g_m_a_t_c_h___t (even if _n_m_a_t_c_h is 0 or REG_NOSUB was
specified), to hold the input offsets for REG_STARTEND.
Use for output is still entirely controlled by _n_m_a_t_c_h; if
_n_m_a_t_c_h is 0 or REG_NOSUB was specified, the value of
_p_m_a_t_c_h[0] will not be changed by a successful _r_e_g_e_x_e_c.
_R_e_g_e_r_r_o_r maps a non-zero _e_r_r_c_o_d_e from either _r_e_g_c_o_m_p or
_r_e_g_e_x_e_c to a human-readable, printable message. If _p_r_e_g
is non-NULL, the error code should have arisen from use of
the _r_e_g_e_x___t pointed to by _p_r_e_g, and if the error code came
from _r_e_g_c_o_m_p, it should have been the result from the most
recent _r_e_g_c_o_m_p using that _r_e_g_e_x___t. (_R_e_g_e_r_r_o_r may be able
to supply a more detailed message using information from
the _r_e_g_e_x___t.) _R_e_g_e_r_r_o_r places the NUL-terminated message
into the buffer pointed to by _e_r_r_b_u_f, limiting the length
(including the NUL) to at most _e_r_r_b_u_f___s_i_z_e bytes. If the
whole message won't 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 _e_r_r_b_u_f___s_i_z_e
is 0, _e_r_r_b_u_f is ignored but the return value is still cor-
rect.
If the _e_r_r_c_o_d_e given to _r_e_g_e_r_r_o_r is first ORed with
REG_ITOA, the ``message'' that results is the printable
name of the error code, e.g. ``REG_NOMATCH'', rather than
an explanation thereof. If _e_r_r_c_o_d_e is REG_ATOI, then _p_r_e_g
shall be non-NULL and the _r_e___e_n_d_p member of the structure
it points to must point to the printable name of an error
code; in this case, the result in _e_r_r_b_u_f is the decimal
digits of the numeric value of the error code (0 if the
name is not recognized). REG_ITOA and REG_ATOI 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
March 20, 1994 4
REGEX(3) REGEX(3)
that they are considered experimental and changes are pos-
sible.
_R_e_g_f_r_e_e frees any dynamically-allocated storage associated
with the compiled RE pointed to by _p_r_e_g. The remaining
_r_e_g_e_x___t is no longer a valid compiled RE and the effect of
supplying it to _r_e_g_e_x_e_c or _r_e_g_e_r_r_o_r is undefined.
None of these functions references global variables except
for tables of constants; all are safe for use from multi-
ple threads if the arguments are safe.
IIMMPPLLEEMMEENNTTAATTIIOONN CCHHOOIICCEESS
There are a number of decisions that 1003.2 leaves up to
the implementor, either by explicitly saying ``undefined''
or by virtue of them being forbidden by the RE grammar.
This implementation treats them as follows.
See _r_e___f_o_r_m_a_t(7) for a discussion of the definition of
case-independent matching.
There is no particular limit on the length of REs, except
insofar as memory is limited. Memory usage is approxi-
mately 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 char-
acter.
Any unmatched [ is a REG_EBRACK error.
Equivalence classes cannot begin or end bracket-expression
ranges. The endpoint of one range cannot begin another.
RE_DUP_MAX, the limit on repetition counts in bounded rep-
etitions, is 255.
A repetition operator (?, *, +, or bounds) cannot follow
another repetition operator. A repetition operator cannot
begin an expression or subexpression or follow `^' or `|'.
`|' cannot appear first or last in a (sub)expression or
after another `|', i.e. an operand of `|' cannot be an
empty subexpression. An empty parenthesized subexpres-
sion, `()', is legal and matches an empty (sub)string. An
empty string is not a legal RE.
A `{' followed by a digit is considered the beginning of
bounds for a bounded repetition, which must then follow
the syntax for bounds. A `{' _n_o_t followed by a digit is
March 20, 1994 5
REGEX(3) REGEX(3)
considered an ordinary character.
`^' and `$' beginning and ending subexpressions in obso-
lete (``basic'') REs are anchors, not ordinary characters.
SSEEEE AALLSSOO
grep(1), re_format(7)
POSIX 1003.2, sections 2.8 (Regular Expression Notation)
and B.5 (C Binding for Regular Expression Matching).
DDIIAAGGNNOOSSTTIICCSS
Non-zero error codes from _r_e_g_c_o_m_p and _r_e_g_e_x_e_c 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, e.g. negative-length string
HHIISSTTOORRYY
Originally written by Henry Spencer. Altered for inclu-
sion in the 4.4BSD distribution.
BBUUGGSS
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, and only
the collating elements etc. of that locale are available.
The back-reference code is subtle and doubts linger about
its correctness in complex cases.
_R_e_g_e_x_e_c performance is poor. This will improve with later
releases. _N_m_a_t_c_h exceeding 0 is expensive; _n_m_a_t_c_h exceed-
ing 1 is worse. _R_e_g_e_x_e_c is largely insensitive to RE com-
plexity _e_x_c_e_p_t that back references are massively expen-
sive. RE length does matter; in particular, there is a
strong speed bonus for keeping RE length under about 30
March 20, 1994 6
REGEX(3) REGEX(3)
characters, with most special characters counting roughly
double.
_R_e_g_c_o_m_p implements bounded repetitions by macro expansion,
which is costly in time and space if counts are large or
bounded repetitions are nested. An RE like, say,
`((((a{1,100}){1,100}){1,100}){1,100}){1,100}' will (even-
tually) run almost any existing machine out of swap space.
There are suspected problems with response to obscure
error conditions. Notably, certain kinds of internal
overflow, produced only by truly enormous REs or by multi-
ply 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 pres-
ence 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 a bit of a
kludge, and bugs may lurk in combinations of word-boundary
matching and anchoring.
March 20, 1994 7
