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Here we describe how you use the Regex data structures and functions in C programs. Regex has three interfaces: one designed for GNU, one compatible with POSIX and one compatible with Berkeley UNIX.
If you're writing code that doesn't need to be compatible with either POSIX or Berkeley UNIX, you can use these functions. They provide more options than the other interfaces.
To compile, match, or search for a given regular expression, you must supply a pattern buffer. A pattern buffer holds one compiled regular expression.(4)
You can have several different pattern buffers simultaneously, each holding a compiled pattern for a different regular expression.
`regex.h' defines the pattern buffer struct as follows:
/* Space that holds the compiled pattern. It is declared as
`unsigned char *' because its elements are
sometimes used as array indexes. */
unsigned char *buffer;
/* Number of bytes to which `buffer' points. */
unsigned long allocated;
/* Number of bytes actually used in `buffer'. */
unsigned long used;
/* Syntax setting with which the pattern was compiled. */
reg_syntax_t syntax;
/* Pointer to a fastmap, if any, otherwise zero. re_search uses
the fastmap, if there is one, to skip over impossible
starting points for matches. */
char *fastmap;
/* Either a translate table to apply to all characters before
comparing them, or zero for no translation. The translation
is applied to a pattern when it is compiled and to a string
when it is matched. */
char *translate;
/* Number of subexpressions found by the compiler. */
size_t re_nsub;
/* Zero if this pattern cannot match the empty string, one else.
Well, in truth it's used only in `re_search_2', to see
whether or not we should use the fastmap, so we don't set
this absolutely perfectly; see `re_compile_fastmap' (the
`duplicate' case). */
unsigned can_be_null : 1;
/* If REGS_UNALLOCATED, allocate space in the `regs' structure
for `max (RE_NREGS, re_nsub + 1)' groups.
If REGS_REALLOCATE, reallocate space if necessary.
If REGS_FIXED, use what's there. */
#define REGS_UNALLOCATED 0
#define REGS_REALLOCATE 1
#define REGS_FIXED 2
unsigned regs_allocated : 2;
/* Set to zero when `regex_compile' compiles a pattern; set to one
by `re_compile_fastmap' if it updates the fastmap. */
unsigned fastmap_accurate : 1;
/* If set, `re_match_2' does not return information about
subexpressions. */
unsigned no_sub : 1;
/* If set, a beginning-of-line anchor doesn't match at the
beginning of the string. */
unsigned not_bol : 1;
/* Similarly for an end-of-line anchor. */
unsigned not_eol : 1;
/* If true, an anchor at a newline matches. */
unsigned newline_anchor : 1;
In GNU, you can both match and search for a given regular expression. To do either, you must first compile it in a pattern buffer (see section GNU Pattern Buffers).
Regular expressions match according to the syntax with which they were
compiled; with GNU, you indicate what syntax you want by setting
the variable re_syntax_options (declared in `regex.h' and
defined in `regex.c') before calling the compiling function,
re_compile_pattern (see below). See section Syntax Bits, and
section Predefined Syntaxes.
You can change the value of re_syntax_options at any time.
Usually, however, you set its value once and then never change it.
re_compile_pattern takes a pattern buffer as an argument. You
must initialize the following fields:
translate initialization
translate
fastmap
buffer
allocated
re_compile_pattern to allocate memory for the
compiled pattern, set both of these to zero. If you have an existing
block of memory (allocated with malloc) you want Regex to use,
set buffer to its address and allocated to its size (in
bytes).
re_compile_pattern uses realloc to extend the space for
the compiled pattern as necessary.
To compile a pattern buffer, use:
char *
re_compile_pattern (const char *regex, const int regex_size,
struct re_pattern_buffer *pattern_buffer)
regex is the regular expression's address, regex_size is its length, and pattern_buffer is the pattern buffer's address.
If re_compile_pattern successfully compiles the regular
expression, it returns zero and sets *pattern_buffer to the
compiled pattern. It sets the pattern buffer's fields as follows:
buffer
used
buffer occupies.
syntax
re_syntax_options.
re_nsub
fastmap_accurate
buffer; in that case (since
you can't make a fastmap without a compiled pattern),
fastmap would either contain an incompatible fastmap, or nothing
at all.
If re_compile_pattern can't compile regex, it returns an
error string corresponding to one of the errors listed in section POSIX Regular Expression Compiling.
Matching the GNU way means trying to match as much of a string as possible starting at a position within it you specify. Once you've compiled a pattern into a pattern buffer (see section GNU Regular Expression Compiling), you can ask the matcher to match that pattern against a string using:
int
re_match (struct re_pattern_buffer *pattern_buffer,
const char *string, const int size,
const int start, struct re_registers *regs)
pattern_buffer is the address of a pattern buffer containing a compiled pattern. string is the string you want to match; it can contain newline and null characters. size is the length of that string. start is the string index at which you want to begin matching; the first character of string is at index zero. See section Using Registers, for a explanation of regs; you can safely pass zero.
re_match matches the regular expression in pattern_buffer
against the string string according to the syntax in
pattern_buffers's syntax field. (See section GNU Regular Expression Compiling, for how to set it.) The function returns
@math{-1} if the compiled pattern does not match any part of
string and @math{-2} if an internal error happens; otherwise, it
returns how many (possibly zero) characters of string the pattern
matched.
An example: suppose pattern_buffer points to a pattern buffer
containing the compiled pattern for `a*', and string points
to `aaaaab' (whereupon size should be 6). Then if start
is 2, re_match returns 3, i.e., `a*' would have matched the
last three `a's in string. If start is 0,
re_match returns 5, i.e., `a*' would have matched all the
`a's in string. If start is either 5 or 6, it returns
zero.
If start is not between zero and size, then
re_match returns @math{-1}.
Searching means trying to match starting at successive positions
within a string. The function re_search does this.
Before calling re_search, you must compile your regular
expression. See section GNU Regular Expression Compiling.
Here is the function declaration:
int
re_search (struct re_pattern_buffer *pattern_buffer,
const char *string, const int size,
const int start, const int range,
struct re_registers *regs)
whose arguments are the same as those to re_match (see section GNU Matching) except that the two arguments start and range
replace re_match's argument start.
If range is positive, then re_search attempts a match
starting first at index start, then at @math{start + 1} if
that fails, and so on, up to @math{start + range}; if
range is negative, then it attempts a match starting first at
index start, then at @math{start -1} if that fails, and so
on.
If start is not between zero and size, then re_search
returns @math{-1}. When range is positive, re_search
adjusts range so that @math{start + range - 1} is
between zero and size, if necessary; that way it won't search
outside of string. Similarly, when range is negative,
re_search adjusts range so that @math{start +
range + 1} is between zero and size, if necessary.
If the fastmap field of pattern_buffer is zero,
re_search matches starting at consecutive positions; otherwise,
it uses fastmap to make the search more efficient.
See section Searching with Fastmaps.
If no match is found, re_search returns @math{-1}. If
a match is found, it returns the index where the match began. If an
internal error happens, it returns @math{-2}.
Using the functions re_match_2 and re_search_2, you can
match or search in data that is divided into two strings.
int
re_match_2 (struct re_pattern_buffer *buffer,
const char *string1, const int size1,
const char *string2, const int size2,
const int start,
struct re_registers *regs,
const int stop)
is similar to re_match (see section GNU Matching) except that you
pass two data strings and sizes, and an index stop beyond
which you don't want the matcher to try matching. As with
re_match, if it succeeds, re_match_2 returns how many
characters of string it matched. Regard string1 and
string2 as concatenated when you set the arguments start and
stop and use the contents of regs; re_match_2 never
returns a value larger than @math{size1 + size2}.
int
re_search_2 (struct re_pattern_buffer *buffer,
const char *string1, const int size1,
const char *string2, const int size2,
const int start, const int range,
struct re_registers *regs,
const int stop)
is similarly related to re_search.
If you're searching through a long string, you should use a fastmap. Without one, the searcher tries to match at consecutive positions in the string. Generally, most of the characters in the string could not start a match. It takes much longer to try matching at a given position in the string than it does to check in a table whether or not the character at that position could start a match. A fastmap is such a table.
More specifically, a fastmap is an array indexed by the characters in
your character set. Under the ASCII encoding, therefore, a fastmap
has 256 elements. If you want the searcher to use a fastmap with a
given pattern buffer, you must allocate the array and assign the array's
address to the pattern buffer's fastmap field. You either can
compile the fastmap yourself or have re_search do it for you;
when fastmap is nonzero, it automatically compiles a fastmap the
first time you search using a particular compiled pattern.
To compile a fastmap yourself, use:
int re_compile_fastmap (struct re_pattern_buffer *pattern_buffer)
pattern_buffer is the address of a pattern buffer. If the
character c could start a match for the pattern,
re_compile_fastmap makes
pattern_buffer->fastmap[c] nonzero. It returns
@math{0} if it can compile a fastmap and @math{-2} if there is an
internal error. For example, if `|' is the alternation operator
and pattern_buffer holds the compiled pattern for `a|b', then
re_compile_fastmap sets fastmap['a'] and
fastmap['b'] (and no others).
re_search uses a fastmap as it moves along in the string: it
checks the string's characters until it finds one that's in the fastmap.
Then it tries matching at that character. If the match fails, it
repeats the process. So, by using a fastmap, re_search doesn't
waste time trying to match at positions in the string that couldn't
start a match.
If you don't want re_search to use a fastmap,
store zero in the fastmap field of the pattern buffer before
calling re_search.
Once you've initialized a pattern buffer's fastmap field, you
need never do so again--even if you compile a new pattern in
it--provided the way the field is set still reflects whether or not you
want a fastmap. re_search will still either do nothing if
fastmap is null or, if it isn't, compile a new fastmap for the
new pattern.
If you set the translate field of a pattern buffer to a translate
table, then the GNU Regex functions to which you've passed that
pattern buffer use it to apply a simple transformation
to all the regular expression and string characters at which they look.
A translate table is an array indexed by the characters in your
character set. Under the ASCII encoding, therefore, a translate
table has 256 elements. The array's elements are also characters in
your character set. When the Regex functions see a character c,
they use translate[c] in its place, with one exception: the
character after a `\' is not translated. (This ensures that, the
operators, e.g., `\B' and `\b', are always distinguishable.)
For example, a table that maps all lowercase letters to the
corresponding uppercase ones would cause the matcher to ignore
differences in case.(5) Such a table would map all characters except lowercase letters
to themselves, and lowercase letters to the corresponding uppercase
ones. Under the ASCII encoding, here's how you could initialize
such a table (we'll call it case_fold):
for (i = 0; i < 256; i++)
case_fold[i] = i;
for (i = 'a'; i <= 'z'; i++)
case_fold[i] = i - ('a' - 'A');
You tell Regex to use a translate table on a given pattern buffer by
assigning that table's address to the translate field of that
buffer. If you don't want Regex to do any translation, put zero into
this field. You'll get weird results if you change the table's contents
anytime between compiling the pattern buffer, compiling its fastmap, and
matching or searching with the pattern buffer.
A group in a regular expression can match a (posssibly empty) substring of the string that regular expression as a whole matched. The matcher remembers the beginning and end of the substring matched by each group.
To find out what they matched, pass a nonzero regs argument to a GNU matching or searching function (see section GNU Matching and section GNU Searching), i.e., the address of a structure of this type, as defined in `regex.h':
struct re_registers
{
unsigned num_regs;
regoff_t *start;
regoff_t *end;
};
Except for (possibly) the num_regs'th element (see below), the
ith element of the start and end arrays records
information about the ith group in the pattern. (They're declared
as C pointers, but this is only because not all C compilers accept
zero-length arrays; conceptually, it is simplest to think of them as
arrays.)
The start and end arrays are allocated in various ways,
depending on the value of the regs_allocated
field in the pattern buffer passed to the matcher.
The simplest and perhaps most useful is to let the matcher (re)allocate
enough space to record information for all the groups in the regular
expression. If regs_allocated is REGS_UNALLOCATED,
the matcher allocates @math{1 + re_nsub} (another field in the
pattern buffer; see section GNU Pattern Buffers). The extra element is set
to @math{-1}, and sets regs_allocated to REGS_REALLOCATE.
Then on subsequent calls with the same pattern buffer and regs
arguments, the matcher reallocates more space if necessary.
It would perhaps be more logical to make the regs_allocated field
part of the re_registers structure, instead of part of the
pattern buffer. But in that case the caller would be forced to
initialize the structure before passing it. Much existing code doesn't
do this initialization, and it's arguably better to avoid it anyway.
re_compile_pattern sets regs_allocated to
REGS_UNALLOCATED,
so if you use the GNU regular expression
functions, you get this behavior by default.
xx document re_set_registers
POSIX, on the other hand, requires a different interface: the
caller is supposed to pass in a fixed-length array which the matcher
fills. Therefore, if regs_allocated is REGS_FIXED
the matcher simply fills that array.
The following examples illustrate the information recorded in the
re_registers structure. (In all of them, `(' represents the
open-group and `)' the close-group operator. The first character
in the string string is at index 0.)
regs->start[i] to the index in string where
the substring matched by the i-th group begins, and
regs->end[i] to the index just beyond that
substring's end. The function sets regs->start[0] and
regs->end[0] to analogous information about the entire
pattern.
For example, when you match `((a)(b))' against `ab', you get:
regs->start[0] and 2 in regs->end[0]
regs->start[1] and 2 in regs->end[1]
regs->start[2] and 1 in regs->end[2]
regs->start[3] and 2 in regs->end[3]
For example, when you match the pattern `(a)*' against the string `aa', you get:
regs->start[0] and 2 in regs->end[0]
regs->start[1] and 2 in regs->end[1]
regs->start[i] and
regs->end[i] to @math{-1}.
For example, when you match the pattern `(a)*b' against the string `b', you get:
regs->start[0] and 1 in regs->end[0]
regs->start[1] and @math{-1} in regs->end[1]
regs->start[i] and
regs->end[i] to the index just beyond that
zero-length string.
For example, when you match the pattern `(a*)b' against the string `b', you get:
regs->start[0] and 1 in regs->end[0]
regs->start[1] and 0 in regs->end[1]
regs->start[j] and
regs->end[j] the last match (if it matched) of
the j-th group.
For example, when you match the pattern `((a*)b)*' against the string `abb', group 2 last matches the empty string, so you get what it previously matched:
regs->start[0] and 3 in regs->end[0]
regs->start[1] and 3 in regs->end[1]
regs->start[2] and 2 in regs->end[2]
When you match the pattern `((a)*b)*' against the string `abb', group 2 doesn't participate in the last match, so you get:
regs->start[0] and 3 in regs->end[0]
regs->start[1] and 3 in regs->end[1]
regs->start[2] and 1 in regs->end[2]
regs->start[i] and
regs->end[i] to @math{-1}, then it also sets
regs->start[j] and
regs->end[j] to @math{-1}.
For example, when you match the pattern `((a)*b)*c' against the string `c', you get:
regs->start[0] and 1 in regs->end[0]
regs->start[1] and @math{-1} in regs->end[1]
regs->start[2] and @math{-1} in regs->end[2]
To free any allocated fields of a pattern buffer, you can use the
POSIX function described in section Freeing POSIX Pattern Buffers,
since the type regex_t---the type for POSIX pattern
buffers--is equivalent to the type re_pattern_buffer. After
freeing a pattern buffer, you need to again compile a regular expression
in it (see section GNU Regular Expression Compiling) before passing it to
a matching or searching function.
If you're writing code that has to be POSIX compatible, you'll need to use these functions. Their interfaces are as specified by POSIX, draft 1003.2/D11.2.
To compile or match a given regular expression the POSIX way, you
must supply a pattern buffer exactly the way you do for GNU
(see section GNU Pattern Buffers). POSIX pattern buffers have type
regex_t, which is equivalent to the GNU pattern buffer
type re_pattern_buffer.
With POSIX, you can only search for a given regular expression; you
can't match it. To do this, you must first compile it in a
pattern buffer, using regcomp.
To compile a pattern buffer, use:
int regcomp (regex_t *preg, const char *regex, int cflags)
preg is the initialized pattern buffer's address, regex is the regular expression's address, and cflags is the compilation flags, which Regex considers as a collection of bits. Here are the valid bits, as defined in `regex.h':
REG_EXTENDED
regcomp sets preg's syntax field accordingly.
REG_ICASE
regcomp sets preg's translate
field to a translate table which ignores case, replacing anything you've
put there before.
REG_NOSUB
no_sub field; see section POSIX Matching,
for what this means.
REG_NEWLINE
.)) doesn't match a newline.
[ ... ] and [^ ... ])) matches a newline.
^)) matches the empty string immediately after a newline,
regardless of how REG_NOTBOL is set (see section POSIX Matching, for
an explanation of REG_NOTBOL).
^)) matches the empty string immediately before a newline,
regardless of how REG_NOTEOL is set (see section POSIX Matching,
for an explanation of REG_NOTEOL).
If regcomp successfully compiles the regular expression, it
returns zero and sets *pattern_buffer to the compiled
pattern. Except for syntax (which it sets as explained above), it
also sets the same fields the same way as does the GNU compiling
function (see section GNU Regular Expression Compiling).
If regcomp can't compile the regular expression, it returns one
of the error codes listed here. (Except when noted differently, the
syntax of in all examples below is basic regular expression syntax.)
REG_BADRPT
REG_BADBR
REG_EBRACE
REG_EBRACK
REG_ERANGE
REG_ECTYPE
REG_EPAREN
REG_ESUBREG
REG_EEND
REG_EESCAPE
REG_BADPAT
REG_ESIZE
REG_ESPACE
Matching the POSIX way means trying to match a null-terminated string starting at its first character. Once you've compiled a pattern into a pattern buffer (see section POSIX Regular Expression Compiling), you can ask the matcher to match that pattern against a string using:
int
regexec (const regex_t *preg, const char *string,
size_t nmatch, regmatch_t pmatch[], int eflags)
preg is the address of a pattern buffer for a compiled pattern. string is the string you want to match.
See section Using Byte Offsets, for an explanation of pmatch. If you
pass zero for nmatch or you compiled preg with the
compilation flag REG_NOSUB set, then regexec will ignore
pmatch; otherwise, you must allocate it to have at least
nmatch elements. regexec will record nmatch byte
offsets in pmatch, and set to @math{-1} any unused elements up to
@math{pmatch[nmatch] - 1}.
eflags specifies execution flags---namely, the two bits
REG_NOTBOL and REG_NOTEOL (defined in `regex.h'). If
you set REG_NOTBOL, then the match-beginning-of-line operator
(see section The Match-beginning-of-line Operator (^)) always fails to match.
This lets you match against pieces of a line, as you would need to if,
say, searching for repeated instances of a given pattern in a line; it
would work correctly for patterns both with and without
match-beginning-of-line operators. REG_NOTEOL works analogously
for the match-end-of-line operator (see section The Match-end-of-line Operator ($)); it exists for symmetry.
regexec tries to find a match for preg in string
according to the syntax in preg's syntax field.
(See section POSIX Regular Expression Compiling, for how to set it.) The
function returns zero if the compiled pattern matches string and
REG_NOMATCH (defined in `regex.h') if it doesn't.
If either regcomp or regexec fail, they return a nonzero
error code, the possibilities for which are defined in `regex.h'.
See section POSIX Regular Expression Compiling, and section POSIX Matching, for
what these codes mean. To get an error string corresponding to these
codes, you can use:
size_t
regerror (int errcode,
const regex_t *preg,
char *errbuf,
size_t errbuf_size)
errcode is an error code, preg is the address of the pattern buffer which provoked the error, errbuf is the error buffer, and errbuf_size is errbuf's size.
regerror returns the size in bytes of the error string
corresponding to errcode (including its terminating null). If
errbuf and errbuf_size are nonzero, it also returns in
errbuf the first @math{errbuf_size - 1} characters of the
error string, followed by a null.
errbuf_size must be a nonnegative number less than or equal to the
size in bytes of errbuf.
You can call regerror with a null errbuf and a zero
errbuf_size to determine how large errbuf need be to
accommodate regerror's error string.
In POSIX, variables of type regmatch_t hold analogous
information, but are not identical to, GNU's registers (see section Using Registers). To get information about registers in POSIX, pass to
regexec a nonzero pmatch of type regmatch_t, i.e.,
the address of a structure of this type, defined in
`regex.h':
typedef struct
{
regoff_t rm_so;
regoff_t rm_eo;
} regmatch_t;
When reading in section Using Registers, about how the matching function
stores the information into the registers, substitute pmatch for
regs, pmatch[i]->rm_so for
regs->start[i] and
pmatch[i]->rm_eo for
regs->end[i].
To free any allocated fields of a pattern buffer, use:
void regfree (regex_t *preg)
preg is the pattern buffer whose allocated fields you want freed.
regfree also sets preg's allocated and used
fields to zero. After freeing a pattern buffer, you need to again
compile a regular expression in it (see section POSIX Regular Expression Compiling) before passing it to the matching function (see section POSIX Matching).
If you're writing code that has to be Berkeley UNIX compatible, you'll need to use these functions whose interfaces are the same as those in Berkeley UNIX.
With Berkeley UNIX, you can only search for a given regular
expression; you can't match one. To search for it, you must first
compile it. Before you compile it, you must indicate the regular
expression syntax you want it compiled according to by setting the
variable re_syntax_options (declared in `regex.h' to some
syntax (see section Regular Expression Syntax).
To compile a regular expression use:
char * re_comp (char *regex)
regex is the address of a null-terminated regular expression.
re_comp uses an internal pattern buffer, so you can use only the
most recently compiled pattern buffer. This means that if you want to
use a given regular expression that you've already compiled--but it
isn't the latest one you've compiled--you'll have to recompile it. If
you call re_comp with the null string (not the empty
string) as the argument, it doesn't change the contents of the pattern
buffer.
If re_comp successfully compiles the regular expression, it
returns zero. If it can't compile the regular expression, it returns
an error string. re_comp's error messages are identical to those
of re_compile_pattern (see section GNU Regular Expression Compiling).
Searching the Berkeley UNIX way means searching in a string
starting at its first character and trying successive positions within
it to find a match. Once you've compiled a pattern using re_comp
(see section BSD Regular Expression Compiling), you can ask Regex
to search for that pattern in a string using:
int re_exec (char *string)
string is the address of the null-terminated string in which you want to search.
re_exec returns either 1 for success or 0 for failure. It
automatically uses a GNU fastmap (see section Searching with Fastmaps).
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