Manual Pages for UNIX Darwin command on man regcomp

REGEX(3) BSD Library Functions Manual REGEX(3)

NAME

rreeggccoommpp, rreeggeexxeecc, rreeggeerrrroorr, rreeggffrreeee - regular-expression library

LLIIBBRRAARRYY

Standard C Library (libc, -lc)

SYNOPSIS

##iinncclluuddee <>

int rreeggccoommpp(regext * restrict preg, const char * restrict pattern, int cflags); int rreeggeexxeecc(const regext * restrict preg, const char * restrict string, sizet nmatch, regmatcht pmatch[restrict], int eflags); sizet rreeggeerrrroorr(int errcode, const regext * restrict preg, char * restrict errbuf, sizet errbufsize); void rreeggffrreeee(regext *preg);

DESCRIPTION

These routines implement IEEE Std 1003.2 (``POSIX.2'') regular expres-

sions (``RE''s); see reformat(7). The rreeggccoommpp() function compiles an RE

written as a string into an internal form, rreeggeexxeecc() matches that inter-

nal form against a string and reports results, rreeggeerrrroorr() transforms

error codes from either into human-readable messages, and rreeggffrreeee() frees

any dynamically-allocated storage used by the internal form of an RE.

The header declares two structure types, regext and regmatcht, the former for compiled internal forms and the latter for match reporting. It also declares the four functions, a type regofft, and a number of constants with names starting with ``REG''. The rreeggccoommpp() function compiles the regular expression contained in the pattern string, subject to the flags in cflags, and places the results in the regext structure pointed to by preg. The cflags argument is the bitwise OR of zero or more of the following flags: REGEXTENDED Compile modern (``extended'') REs, rather than the obsolete (``basic'') REs that are the default. REGBASIC This is a synonym for 0, provided as a counterpart to REGEXTENDED to improve readability. REGNOSPEC 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, compati-

ble with but not specified by IEEE Std 1003.2 (``POSIX.2''), and should be used with caution in software intended to be portable to other systems. REGEXTENDED and REGNOSPEC may not be used in the same call to rreeggccoommpp().

REGICASE Compile for matching that ignores upper/lower case distinc-

tions. See reformat(7).

REGNOSUB Compile for matching that need only report success or fail-

ure, not what was matched.

REGNEWLINE Compile for newline-sensitive matching. By default, new-

line is a completely ordinary character with no special meaning in either REs or strings. With this flag, `[^' bracket expressions and `.' never match newline, 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. REGPEND The regular expression ends, not at the first NUL, but just before the character pointed to by the reendp member of the structure pointed to by preg. The reendp member is of type const char *. This flag permits inclusion of NULs in the RE; they are considered ordinary characters. This is an extension, compatible with but not specified by IEEE Std 1003.2 (``POSIX.2''), and should be used with caution in software intended to be portable to other systems. When successful, rreeggccoommpp() returns 0 and fills in the structure pointed

to by preg. One member of that structure (other than reendp) is publi-

cized: rensub, of type sizet, contains the number of parenthesized subexpressions within the RE (except that the value of this member is undefined if the REGNOSUB flag was used). If rreeggccoommpp() fails, it

returns a non-zero error code; see DIAGNOSTICS.

The rreeggeexxeecc() function matches the compiled RE pointed to by preg against the string, subject to the flags in eflags, and reports results using nmatch, pmatch, and the returned value. The RE must have been compiled by a previous invocation of rreeggccoommpp(). The compiled form is not altered

during execution of rreeggeexxeecc(), so a single compiled RE can be used simul-

taneously by multiple threads.

By default, the NUL-terminated string pointed to by string is considered

to be the text of an entire line, minus any terminating newline. The eflags argument is the bitwise OR of zero or more of the following flags: REGNOTBOL 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 REGNEWLINE.

REGNOTEOL The NUL terminating the string does not end a line, so the

`$' anchor should not match before it. This does not

affect the behavior of newlines under REGNEWLINE. REGSTARTEND The string is considered to start at string + pmatch[0].rmso and to have a terminating NUL located at string + pmatch[0].rmeo (there need not actually be a NUL at that location), regardless of the value of nmatch. See below for the definition of pmatch and nmatch. This is an extension, compatible with but not specified by IEEE Std 1003.2 (``POSIX.2''), and should be used with caution in software intended to be portable to other systems. Note

that a non-zero rmso does not imply REGNOTBOL;

REGSTARTEND affects only the location of the string, not how it is matched. See reformat(7) for a discussion of what is matched in situations where an RE or a portion thereof could match any of several substrings of string.

Normally, rreeggeexxeecc() returns 0 for success and the non-zero code

REGNOMATCH for failure. Other non-zero error codes may be returned in

exceptional situations; see DIAGNOSTICS. If REGNOSUB was specified in the compilation of the RE, or if nmatch is 0, rreeggeexxeecc() ignores the pmatch argument (but see below for the case where REGSTARTEND is specified). Otherwise, pmatch points to an array of nmatch structures of type regmatcht. Such a structure has at least the members rmso and rmeo, both of type regofft (a signed arithmetic

type at least as large as an offt and a ssizet), containing respec-

tively 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 string argument given to rreeggeexxeecc(). An empty

substring is denoted by equal offsets, both indicating the character fol-

lowing the empty substring.

The 0th member of the pmatch array is filled in to indicate what sub-

string of string was matched by the entire 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 participate in the match at all, or to subexpressions that do not

exist in the RE (that is, i > preg->rensub)) have both rmso and rmeo

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 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 empties.) If REGSTARTEND is specified, pmatch must point to at least one regmatcht (even if nmatch is 0 or REGNOSUB was specified), to hold the

input offsets for REGSTARTEND. Use for output is still entirely con-

trolled by nmatch; if nmatch is 0 or REGNOSUB was specified, the value of pmatch[0] will not be changed by a successful rreeggeexxeecc().

The rreeggeerrrroorr() function maps a non-zero errcode from either rreeggccoommpp() or

rreeggeexxeecc() to a human-readable, printable message. If preg is non-NULL,

the error code should have arisen from use of the regext pointed to by preg, and if the error code came from rreeggccoommpp(), it should have been the result from the most recent rreeggccoommpp() using that regext. The

(rreeggeerrrroorr() may be able to supply a more detailed message using informa-

tion from the regext.) The rreeggeerrrroorr() function places the NUL-termi-

nated message into the buffer pointed to by errbuf, limiting the length (including the NUL) to at most errbufsize bytes. If the whole message

won't fit, as much of it as will fit before the terminating NUL is sup-

plied. In any case, the returned value is the size of buffer needed to hold the whole message (including terminating NUL). If errbufsize is 0, errbuf is ignored but the return value is still correct. If the errcode given to rreeggeerrrroorr() is first ORed with REGITOA, the ``message'' that results is the printable name of the error code, e.g. ``REGNOMATCH'', rather than an explanation thereof. If errcode is

REGATOI, then preg shall be non-NULL and the reendp member of the

structure it points to must point to the printable name of an error code; in this case, the result in errbuf is the decimal digits of the numeric value of the error code (0 if the name is not recognized). REGITOA and

REGATOI are intended primarily as debugging facilities; they are exten-

sions, compatible with but not specified by IEEE Std 1003.2 (``POSIX.2''), and should be used with caution in software intended to be portable to other systems. Be warned also that they are considered experimental and changes are possible.

The rreeggffrreeee() function frees any dynamically-allocated storage associated

with the compiled RE pointed to by preg. The remaining regext is no longer a valid compiled RE and the effect of supplying it to rreeggeexxeecc() or rreeggeerrrroorr() is undefined. None of these functions references global variables except for tables of constants; all are safe for use from multiple threads if the arguments are safe. IIMMPPLLEEMMEENNTTAATTIIOONN CCHHOOIICCEESS There are a number of decisions that IEEE Std 1003.2 (``POSIX.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 reformat(7) for a discussion of the definition of case-independent

matching.

There is no particular limit on the length of REs, except insofar as mem-

ory is limited. Memory usage is approximately 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 IEEE Std 1003.2 (``POSIX.2'') (such magic meanings occur only in obso-

lete [``basic''] REs) is taken as an ordinary character. Any unmatched `[' is a REGEBRACK error.

Equivalence classes cannot begin or end bracket-expression ranges. The

endpoint of one range cannot begin another. REDUPMAX, the limit on repetition counts in bounded repetitions, 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 subexpression, `()', 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 `{' not followed by a digit is considered an ordinary character.

`^' and `$' beginning and ending subexpressions in obsolete (``basic'')

REs are anchors, not ordinary characters.

BUGS

This is an alpha release with known defects. Please report problems.

The back-reference code is subtle and doubts linger about its correctness

in complex cases. The rreeggeexxeecc() function performance is poor. This will improve with later releases. The nmatch argument exceeding 0 is expensive; nmatch exceeding

1 is worse. The rreeggeexxeecc() function is largely insensitive to RE complex-

ity except that back references are massively expensive. RE length does matter; in particular, there is a strong speed bonus for keeping RE length under about 30 characters, with most special characters counting roughly double. The rreeggccoommpp() function implements bounded repetitions by macro expansion,

which is costly in time and space if counts are large or bounded repeti-

tions are nested. An RE like, say, `((((a{1,100}){1,100}){1,100}){1,100}){1,100}' will (eventually) 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 enor-

mous REs or by multiply nested bounded repetitions, are probably not han-

dled well. Due to a mistake in IEEE Std 1003.2 (``POSIX.2''), things like `a)b' are legal REs because `)' is a special character only in the presence 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, behav-

ior 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.

BSD July 12, 2004 BSD