PCRECPP
Section: C Library Functions (3)
Updated: 08 January 2012
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NAME
PCRE - Perl-compatible regular expressions.
SYNOPSIS OF C++ WRAPPER
#include <pcrecpp.h>
DESCRIPTION
The C++ wrapper for PCRE was provided by Google Inc. Some additional
functionality was added by Giuseppe Maxia. This brief man page was constructed
from the notes in the pcrecpp.h file, which should be consulted for
further details. Note that the C++ wrapper supports only the original 8-bit
PCRE library. There is no 16-bit or 32-bit support at present.
MATCHING INTERFACE
The "FullMatch" operation checks that supplied text matches a supplied pattern
exactly. If pointer arguments are supplied, it copies matched sub-strings that
match sub-patterns into them.
Example: successful match
pcrecpp::RE re("h.*o");
re.FullMatch("hello");
Example: unsuccessful match (requires full match):
pcrecpp::RE re("e");
!re.FullMatch("hello");
Example: creating a temporary RE object:
pcrecpp::RE("h.*o").FullMatch("hello");
You can pass in a "const char*" or a "string" for "text". The examples below
tend to use a const char*. You can, as in the different examples above, store
the RE object explicitly in a variable or use a temporary RE object. The
examples below use one mode or the other arbitrarily. Either could correctly be
used for any of these examples.
You must supply extra pointer arguments to extract matched subpieces.
Example: extracts "ruby" into "s" and 1234 into "i"
int i;
string s;
pcrecpp::RE re("(\\w+):(\\d+)");
re.FullMatch("ruby:1234", &s, &i);
Example: does not try to extract any extra sub-patterns
re.FullMatch("ruby:1234", &s);
Example: does not try to extract into NULL
re.FullMatch("ruby:1234", NULL, &i);
Example: integer overflow causes failure
!re.FullMatch("ruby:1234567891234", NULL, &i);
Example: fails because there aren't enough sub-patterns:
!pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);
Example: fails because string cannot be stored in integer
!pcrecpp::RE("(.*)").FullMatch("ruby", &i);
The provided pointer arguments can be pointers to any scalar numeric
type, or one of:
string (matched piece is copied to string)
StringPiece (StringPiece is mutated to point to matched piece)
T (where "bool T::ParseFrom(const char*, int)" exists)
NULL (the corresponding matched sub-pattern is not copied)
The function returns true iff all of the following conditions are satisfied:
a. "text" matches "pattern" exactly;
b. The number of matched sub-patterns is >= number of supplied
pointers;
c. The "i"th argument has a suitable type for holding the
string captured as the "i"th sub-pattern. If you pass in
void * NULL for the "i"th argument, or a non-void * NULL
of the correct type, or pass fewer arguments than the
number of sub-patterns, "i"th captured sub-pattern is
ignored.
CAVEAT: An optional sub-pattern that does not exist in the matched
string is assigned the empty string. Therefore, the following will
return false (because the empty string is not a valid number):
int number;
pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);
The matching interface supports at most 16 arguments per call.
If you need more, consider using the more general interface
pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for
DoMatch.
NOTE: Do not use no_arg, which is used internally to mark the end of a
list of optional arguments, as a placeholder for missing arguments, as this can
lead to segfaults.
QUOTING METACHARACTERS
You can use the "QuoteMeta" operation to insert backslashes before all
potentially meaningful characters in a string. The returned string, used as a
regular expression, will exactly match the original string.
Example:
string quoted = RE::QuoteMeta(unquoted);
Note that it's legal to escape a character even if it has no special meaning in
a regular expression -- so this function does that. (This also makes it
identical to the perl function of the same name; see "perldoc -f quotemeta".)
For example, "1.5-2.0?" becomes "1\.5\-2\.0\?".
PARTIAL MATCHES
You can use the "PartialMatch" operation when you want the pattern
to match any substring of the text.
Example: simple search for a string:
pcrecpp::RE("ell").PartialMatch("hello");
Example: find first number in a string:
int number;
pcrecpp::RE re("(\\d+)");
re.PartialMatch("x*100 + 20", &number);
assert(number == 100);
UTF-8 AND THE MATCHING INTERFACE
By default, pattern and text are plain text, one byte per character. The UTF8
flag, passed to the constructor, causes both pattern and string to be treated
as UTF-8 text, still a byte stream but potentially multiple bytes per
character. In practice, the text is likelier to be UTF-8 than the pattern, but
the match returned may depend on the UTF8 flag, so always use it when matching
UTF8 text. For example, "." will match one byte normally but with UTF8 set may
match up to three bytes of a multi-byte character.
Example:
pcrecpp::RE_Options options;
options.set_utf8();
pcrecpp::RE re(utf8_pattern, options);
re.FullMatch(utf8_string);
Example: using the convenience function UTF8():
pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
re.FullMatch(utf8_string);
NOTE: The UTF8 flag is ignored if pcre was not configured with the
--enable-utf8 flag.
PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
PCRE defines some modifiers to change the behavior of the regular expression
engine. The C++ wrapper defines an auxiliary class, RE_Options, as a vehicle to
pass such modifiers to a RE class. Currently, the following modifiers are
supported:
modifier description Perl corresponding
PCRE_CASELESS case insensitive match /i
PCRE_MULTILINE multiple lines match /m
PCRE_DOTALL dot matches newlines /s
PCRE_DOLLAR_ENDONLY $ matches only at end N/A
PCRE_EXTRA strict escape parsing N/A
PCRE_EXTENDED ignore white spaces /x
PCRE_UTF8 handles UTF8 chars built-in
PCRE_UNGREEDY reverses * and *? N/A
PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*)
(*) Both Perl and PCRE allow non capturing parentheses by means of the
"?:" modifier within the pattern itself. e.g. (?:ab|cd) does not
capture, while (ab|cd) does.
For a full account on how each modifier works, please check the
PCRE API reference page.
For each modifier, there are two member functions whose name is made
out of the modifier in lowercase, without the "PCRE_" prefix. For
instance, PCRE_CASELESS is handled by
bool caseless()
which returns true if the modifier is set, and
RE_Options & set_caseless(bool)
which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can be
accessed through the set_match_limit() and match_limit() member
functions. Setting match_limit to a non-zero value will limit the
execution of pcre to keep it from doing bad things like blowing the stack or
taking an eternity to return a result. A value of 5000 is good enough to stop
stack blowup in a 2MB thread stack. Setting match_limit to zero disables
match limiting. Alternatively, you can call match_limit_recursion()
which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to limit how much PCRE
recurses. match_limit() limits the number of matches PCRE does;
match_limit_recursion() limits the depth of internal recursion, and
therefore the amount of stack that is used.
Normally, to pass one or more modifiers to a RE class, you declare
a RE_Options object, set the appropriate options, and pass this
object to a RE constructor. Example:
RE_Options opt;
opt.set_caseless(true);
if (RE("HELLO", opt).PartialMatch("hello world")) ...
RE_options has two constructors. The default constructor takes no arguments and
creates a set of flags that are off by default. The optional parameter
option_flags is to facilitate transfer of legacy code from C programs.
This lets you do
RE(pattern,
RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);
However, new code is better off doing
RE(pattern,
RE_Options().set_caseless(true).set_multiline(true))
.PartialMatch(str);
If you are going to pass one of the most used modifiers, there are some
convenience functions that return a RE_Options class with the
appropriate modifier already set: CASELESS(), UTF8(),
MULTILINE(), DOTALL(), and EXTENDED().
If you need to set several options at once, and you don't want to go through
the pains of declaring a RE_Options object and setting several options, there
is a parallel method that give you such ability on the fly. You can concatenate
several set_xxxxx() member functions, since each of them returns a
reference to its class object. For example, to pass PCRE_CASELESS,
PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one statement, you may write:
RE(" ^ xyz \\s+ .* blah$",
RE_Options()
.set_caseless(true)
.set_extended(true)
.set_multiline(true)).PartialMatch(sometext);
SCANNING TEXT INCREMENTALLY
The "Consume" operation may be useful if you want to repeatedly
match regular expressions at the front of a string and skip over
them as they match. This requires use of the "StringPiece" type,
which represents a sub-range of a real string. Like RE, StringPiece
is defined in the pcrecpp namespace.
Example: read lines of the form "var = value" from a string.
string contents = ...; // Fill string somehow
pcrecpp::StringPiece input(contents); // Wrap in a StringPiece
string var;
int value;
pcrecpp::RE re("(\\w+) = (\\d+)\n");
while (re.Consume(&input, &var, &value)) {
...;
}
Each successful call to "Consume" will set "var/value", and also
advance "input" so it points past the matched text.
The "FindAndConsume" operation is similar to "Consume" but does not
anchor your match at the beginning of the string. For example, you
could extract all words from a string by repeatedly calling
pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)
PARSING HEX/OCTAL/C-RADIX NUMBERS
By default, if you pass a pointer to a numeric value, the
corresponding text is interpreted as a base-10 number. You can
instead wrap the pointer with a call to one of the operators Hex(),
Octal(), or CRadix() to interpret the text in another base. The
CRadix operator interprets C-style "0" (base-8) and "0x" (base-16)
prefixes, but defaults to base-10.
Example:
int a, b, c, d;
pcrecpp::RE re("(.*) (.*) (.*) (.*)");
re.FullMatch("100 40 0100 0x40",
pcrecpp::Octal(&a), pcrecpp::Hex(&b),
pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));
will leave 64 in a, b, c, and d.
REPLACING PARTS OF STRINGS
You can replace the first match of "pattern" in "str" with "rewrite".
Within "rewrite", backslash-escaped digits (\1 to \9) can be
used to insert text matching corresponding parenthesized group
from the pattern. \0 in "rewrite" refers to the entire matching
text. For example:
string s = "yabba dabba doo";
pcrecpp::RE("b+").Replace("d", &s);
will leave "s" containing "yada dabba doo". The result is true if the pattern
matches and a replacement occurs, false otherwise.
GlobalReplace is like Replace except that it replaces all
occurrences of the pattern in the string with the rewrite. Replacements are
not subject to re-matching. For example:
string s = "yabba dabba doo";
pcrecpp::RE("b+").GlobalReplace("d", &s);
will leave "s" containing "yada dada doo". It returns the number of
replacements made.
Extract is like Replace, except that if the pattern matches,
"rewrite" is copied into "out" (an additional argument) with substitutions.
The non-matching portions of "text" are ignored. Returns true iff a match
occurred and the extraction happened successfully; if no match occurs, the
string is left unaffected.
AUTHOR
The C++ wrapper was contributed by Google Inc.
Copyright (c) 2007 Google Inc.
REVISION
Last updated: 08 January 2012
Index
- NAME
-
- SYNOPSIS OF C++ WRAPPER
-
- DESCRIPTION
-
- MATCHING INTERFACE
-
- QUOTING METACHARACTERS
-
- PARTIAL MATCHES
-
- UTF-8 AND THE MATCHING INTERFACE
-
- PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
-
- SCANNING TEXT INCREMENTALLY
-
- PARSING HEX/OCTAL/C-RADIX NUMBERS
-
- REPLACING PARTS OF STRINGS
-
- AUTHOR
-
- REVISION
-
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