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regcomp regexec regerror regfree - regular-expression library
Lb libc
The header
#include <regex.h>
declares two structure types,
regex_t
and
regmatch_t ,
the former for compiled internal forms and the latter for match reporting.
It also declares the four functions,
a type
regoff_t ,
and a number of constants with names starting with
``REG_
''
The
regcomp ();
function
compiles the regular expression contained in the
Fa pattern
string,
subject to the flags in
Fa cflags ,
and places the results in the
regex_t
structure pointed to by
Fa preg .
The
Fa cflags
argument
is the bitwise OR of zero or more of the following flags:
When successful,
regcomp ();
returns 0 and fills in the structure pointed to by
Fa preg .
One member of that structure
(other than
re_endp
is publicized:
re_nsub
of type
size_t ,
contains the number of parenthesized subexpressions within the RE
(except that the value of this member is undefined if the
REG_NOSUB
flag was used).
If
regcomp ();
fails, it returns a non-zero error code;
see
Sx DIAGNOSTICS .
The
regexec ();
function
matches the compiled RE pointed to by
Fa preg
against the
Fa string ,
subject to the flags in
Fa eflags ,
and reports results using
Fa nmatch ,
Fa pmatch ,
and the returned value.
The RE must have been compiled by a previous invocation of
regcomp (.);
The compiled form is not altered during execution of
regexec (,);
so a single compiled RE can be used simultaneously by multiple threads.
By default, the NUL-terminated string pointed to by Fa string is considered to be the text of an entire line, minus any terminating newline. The Fa eflags argument is the bitwise OR of zero or more of the following flags:
See re_format7 for a discussion of what is matched in situations where an RE or a portion thereof could match any of several substrings of Fa string .
Normally,
regexec ();
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
Sx DIAGNOSTICS .
If
REG_NOSUB
was specified in the compilation of the RE,
or if
Fa nmatch
is 0,
regexec ();
ignores the
Fa pmatch
argument (but see below for the case where
REG_STARTEND
is specified).
Otherwise,
Fa pmatch
points to an array of
Fa nmatch
structures of type
regmatch_t .
Such a structure has at least the members
rm_so
and
rm_eo
both of type
regoff_t
(a signed arithmetic type at least as large as an
off_t
and a
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
regexec (.);
An empty substring is denoted by equal offsets,
both indicating the character following the empty substring.
The 0th member of the Fa pmatch array is filled in to indicate what substring of Fa 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 > Fa preg Ns -> Ns Va re_nsub ) ) have both rm_so and 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 RE `(b*)+' matches `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 `b' , so the reported substring is one of the empties.)
If
REG_STARTEND
is specified,
Fa pmatch
must point to at least one
regmatch_t
(even if
Fa nmatch
is 0 or
REG_NOSUB
was specified),
to hold the input offsets for
REG_STARTEND
Use for output is still entirely controlled by
Fa nmatch ;
if
Fa nmatch
is 0 or
REG_NOSUB
was specified,
the value of
Fa pmatch Ns [0]
will not be changed by a successful
regexec (.);
The
regerror ();
function
maps a non-zero
Fa errcode
from either
regcomp ();
or
regexec ();
to a human-readable, printable message.
If
Fa preg
is
non- NULL
the error code should have arisen from use of
the
regex_t
pointed to by
Fa preg ,
and if the error code came from
regcomp (,);
it should have been the result from the most recent
regcomp ();
using that
regex_t .
The
Fn ( regerror
may be able to supply a more detailed message using information
from the
regex_t . )
The
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.
If the
Fa errcode
given to
regerror ();
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
Fa errcode
is
REG_ATOI
then
Fa preg
shall be
non- NULL
and the
re_endp
member of the structure it points to
must point to the printable name of an error code;
in this case, the result in
Fa errbuf
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
St -p1003.2 ,
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
regfree ();
function
frees any dynamically-allocated storage associated with the compiled RE
pointed to by
Fa preg .
The remaining
regex_t
is no longer a valid compiled RE
and the effect of supplying it to
regexec ();
or
regerror ();
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.
See re_format7 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 approximately linear in RE size, and largely insensitive to RE complexity, except for bounded repetitions. See Sx 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 St -p1003.2 (such magic meanings occur only in obsolete Bq Dq basic REs) is taken as an ordinary character.
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 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.
St -p1003.2 , sections 2.8 (Regular Expression Notation) and B.5 (C Binding for Regular Expression Matching).
The back-reference code is subtle and doubts linger about its correctness in complex cases.
The
regexec ();
function
performance is poor.
This will improve with later releases.
The
Fa nmatch
argument
exceeding 0 is expensive;
Fa nmatch
exceeding 1 is worse.
The
regexec ();
function
is largely insensitive to RE complexity
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
regcomp ();
function
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 (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 enormous REs or by multiply nested bounded repetitions, are probably not handled well.
Due to a mistake in St -p1003.2 , things like `a)b' are legal REs because `)' is a special character only in the presence of a previous unmatched `(' This cannot 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.
Word-boundary matching does not work properly in multibyte locales.
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