These routines implement
regular expressions
(“REs”);
see
re_format(7).
regcomp();
compiles an RE written as a string into an internal form,
regexec();
matches that internal form against a string and reports results,
regerror();
transforms error codes from either into human-readable messages, and
regfree();
frees any dynamically allocated storage used by the internal form
of an RE.
The header
‹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_.
regcomp();
compiles the regular expression contained in the
pattern
string,
subject to the flags in
cflags,
and places the results in the
regex_t
structure pointed to by
preg.
cflags
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 counterpart to
REG_EXTENDED
to improve readability.
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,
compatible with but not specified by
and should be used with
caution in software intended to be portable to other systems.
REG_EXTENDED
and
REG_NOSPEC
may not be used in the same call to
regcomp();.
REG_ICASE
Compile for matching that ignores upper/lower case distinctions.
See
re_format(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 newline,
a
"^"
anchor matches the null string after any newline in the string
in addition to its normal function,
and the
"$"
anchor matches the null string before any newline in the
string in addition to its normal function.
REG_PEND
The regular expression ends,
not at the first NUL,
but just before the character pointed to by the
re_endp
member of the structure pointed to by
preg.
The
re_endp
member is of type
const\char\ *.
This flag permits inclusion of NULs in the RE;
they are considered ordinary characters.
This is an extension,
compatible with but not specified by
and should be used with
caution in software intended to be portable to other systems.
When successful,
regcomp();
returns 0 and fills in the structure pointed to by
preg.
One member of that structure
(other than
re_endp)
is publicized:
re_nsub,
of type
size_t,
contains the number of parenthesized subexpressions within the RE
(except that the value of this member is undefined if the
REG_NOSUB
flag was used).
If
regcomp();
fails, it returns a non-zero error code;
see DIAGNOSTICS.
regexec();
matches the compiled RE pointed to by
preg
against the
string,
subject to the flags in
eflags,
and reports results using
nmatch,
pmatch,
and the returned value.
The RE must have been compiled by a previous invocation of
regcomp();.
The compiled form is not altered during execution of
regexec();,
so a single compiled RE can be used simultaneously by multiple threads.
By default,
the NUL-terminated string pointed to by
string
is considered to be the text of an entire line, minus any terminating
newline.
The
eflags
argument is the bitwise
OR
of zero or more of the following flags:
REG_NOTBOL
The first character of
the string
is not the beginning of a line, so the
"^"
anchor should not match before it.
This does not affect the behavior of newlines under
REG_NEWLINE.
REG_NOTEOL
The NUL terminating
the string
does not end a line, so the
"$"
anchor should not match before it.
This does not affect the behavior of newlines under
REG_NEWLINE.
REG_STARTEND
The string is considered to start at
\fIstring\fR\ + \fIpmatch\fR[0].\fIrm_so\fR
and to have a terminating NUL located at
\fIstring\fR\ + \fIpmatch\fR[0].\fIrm_eo\fR
(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
and should be used with
caution in software intended to be portable to other systems.
Note that a non-zero \fIrm_so\fR does not imply
REG_NOTBOL;
REG_STARTEND
affects only the location of the string,
not how it is matched.
See
re_format(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,
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 DIAGNOSTICS.
If
REG_NOSUB
was specified in the compilation of the RE,
or if
nmatch
is 0,
regexec();
ignores the
pmatch
argument (but see below for the case where
REG_STARTEND
is specified).
Otherwise,
pmatch
points to an array of
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
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
pmatch
array is filled in to indicate what substring 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\(emcorresponding either to subexpressions that
did not participate in the match at all, or to subexpressions that do not
exist in the RE (that is, \fIi\fR\ > \fIpreg\fR\->\fIre_nsub\fR)\(emhave 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
‘b ’
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,
pmatch
must point to at least one
regmatch_t
(even if
nmatch
is 0 or
REG_NOSUB
was specified),
to hold the input offsets for
REG_STARTEND.
Use for output is still entirely controlled by
nmatch;
if
nmatch
is 0 or
REG_NOSUB
was specified,
the value of
pmatch[0]
will not be changed by a successful
regexec();.
regerror();
maps a non-zero
errcode
from either
regcomp();
or
regexec();
to a human-readable, printable message.
If
preg
is non-NULL,
the error code should have arisen from use of
the
regex_t
pointed to by
preg,
and if the error code came from
regcomp();,
it should have been the result from the most recent
regcomp();
using that
regex_t.
.Pf ( Fn regerror
may be able to supply a more detailed message using information
from the
regex_t.)
regerror();
places the NUL-terminated message into the buffer pointed to by
errbuf,
limiting the length (including the NUL) to at most
errbuf_size
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 the terminating NUL).
If
errbuf_size
is 0,
errbuf
is ignored but the return value is still correct.
If the
errcode
given to
regerror();
is first
OR
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
errcode
is
REG_ATOI,
then
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
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
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.
regfree();
frees any dynamically allocated storage associated with the compiled RE
pointed to by
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.
IMPLEMENTATION CHOICES
There are a number of decisions that
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
re_format(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 approximately linear in RE size, and largely insensitive
to RE complexity, except for bounded repetitions.
See
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
(such magic meanings occur only in obsolete 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
"|".
A
"|"
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.
DIAGNOSTICS
Non-zero error codes from
regcomp();
and
regexec();
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
\e 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”
\(emyou found a bug
REG_INVARG
invalid argument, e.g., negative-length string
SEE ALSO
grep(1),
re_format(7)
sections 2.8 (Regular Expression Notation)
and
B.5 (C Binding for Regular Expression Matching).
HISTORY
Originally written by Henry Spencer.
Altered for inclusion in the
4.4BSD
distribution.
BUGS
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
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.
regexec();
performance is poor.
This will improve with later releases.
nmatch
exceeding 0 is expensive;
nmatch
exceeding 1 is worse.
regexec();
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.
regcomp();
implements bounded repetitions by macro expansion,
which is costly in time and space if counts are large
or bounded repetitions are nested.
A 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
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\e(\e(b\e)*\e2\e)*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.