📄 pcre.txt
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with no special meaning is faulted.
(d) If PCRE_UNGREEDY is set, the greediness of the repeti-
tion quantifiers is inverted, that is, by default they are
not greedy, but if followed by a question mark they are.
(e) PCRE_ANCHORED can be used to force a pattern to be tried
only at the start of the subject.
(f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options
for pcre_exec() have no Perl equivalents.
(g) The (?R) construct allows for recursive pattern matching
(Perl 5.6 can do this using the (?p{code}) construct, which
PCRE cannot of course support.)
REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions sup-
ported by PCRE are described below. Regular expressions are
also described in the Perl documentation and in a number of
other books, some of which have copious examples. Jeffrey
Friedl's "Mastering Regular Expressions", published by
O'Reilly (ISBN 1-56592-257), covers them in great detail.
The description here is intended as reference documentation.
The basic operation of PCRE is on strings of bytes. However,
there is the beginnings of some support for UTF-8 character
strings. To use this support you must configure PCRE to
include it, and then call pcre_compile() with the PCRE_UTF8
option. How this affects the pattern matching is described
in the final section of this document.
A regular expression is a pattern that is matched against a
subject string from left to right. Most characters stand for
themselves in a pattern, and match the corresponding charac-
ters in the subject. As a trivial example, the pattern
The quick brown fox
matches a portion of a subject string that is identical to
itself. The power of regular expressions comes from the
ability to include alternatives and repetitions in the pat-
tern. These are encoded in the pattern by the use of meta-
characters, which do not stand for themselves but instead
are interpreted in some special way.
There are two different sets of meta-characters: those that
are recognized anywhere in the pattern except within square
brackets, and those that are recognized in square brackets.
Outside square brackets, the meta-characters are as follows:
\ general escape character with several uses
^ assert start of subject (or line, in multiline
mode)
$ assert end of subject (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
{ start min/max quantifier
Part of a pattern that is in square brackets is called a
"character class". In a character class the only meta-
characters are:
\ general escape character
^ negate the class, but only if the first character
- indicates character range
] terminates the character class
The following sections describe the use of each of the
meta-characters.
BACKSLASH
The backslash character has several uses. Firstly, if it is
followed by a non-alphameric character, it takes away any
special meaning that character may have. This use of
backslash as an escape character applies both inside and
outside character classes.
For example, if you want to match a "*" character, you write
"\*" in the pattern. This applies whether or not the follow-
ing character would otherwise be interpreted as a meta-
character, so it is always safe to precede a non-alphameric
with "\" to specify that it stands for itself. In particu-
lar, if you want to match a backslash, you write "\\".
If a pattern is compiled with the PCRE_EXTENDED option, whi-
tespace in the pattern (other than in a character class) and
characters between a "#" outside a character class and the
next newline character are ignored. An escaping backslash
can be used to include a whitespace or "#" character as part
of the pattern.
A second use of backslash provides a way of encoding non-
printing characters in patterns in a visible manner. There
is no restriction on the appearance of non-printing charac-
ters, apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is
usually easier to use one of the following escape sequences
than the binary character it represents:
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any character
\e escape (hex 1B)
\f formfeed (hex 0C)
\n newline (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\xhh character with hex code hh
\ddd character with octal code ddd, or backreference
The precise effect of "\cx" is as follows: if "x" is a lower
case letter, it is converted to upper case. Then bit 6 of
the character (hex 40) is inverted. Thus "\cz" becomes hex
1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.
After "\x", up to two hexadecimal digits are read (letters
can be in upper or lower case).
After "\0" up to two further octal digits are read. In both
cases, if there are fewer than two digits, just those that
are present are used. Thus the sequence "\0\x\07" specifies
two binary zeros followed by a BEL character. Make sure you
supply two digits after the initial zero if the character
that follows is itself an octal digit.
The handling of a backslash followed by a digit other than 0
is complicated. Outside a character class, PCRE reads it
and any following digits as a decimal number. If the number
is less than 10, or if there have been at least that many
previous capturing left parentheses in the expression, the
entire sequence is taken as a back reference. A description
of how this works is given later, following the discussion
of parenthesized subpatterns.
Inside a character class, or if the decimal number is
greater than 9 and there have not been that many capturing
subpatterns, PCRE re-reads up to three octal digits follow-
ing the backslash, and generates a single byte from the
least significant 8 bits of the value. Any subsequent digits
stand for themselves. For example:
\040 is another way of writing a space
\40 is the same, provided there are fewer than 40
previous capturing subpatterns
\7 is always a back reference
\11 might be a back reference, or another way of
writing a tab
\011 is always a tab
\0113 is a tab followed by the character "3"
\113 is the character with octal code 113 (since there
can be no more than 99 back references)
\377 is a byte consisting entirely of 1 bits
\81 is either a back reference, or a binary zero
followed by the two characters "8" and "1"
Note that octal values of 100 or greater must not be intro-
duced by a leading zero, because no more than three octal
digits are ever read.
All the sequences that define a single byte value can be
used both inside and outside character classes. In addition,
inside a character class, the sequence "\b" is interpreted
as the backspace character (hex 08). Outside a character
class it has a different meaning (see below).
The third use of backslash is for specifying generic charac-
ter types:
\d any decimal digit
\D any character that is not a decimal digit
\s any whitespace character
\S any character that is not a whitespace character
\w any "word" character
\W any "non-word" character
Each pair of escape sequences partitions the complete set of
characters into two disjoint sets. Any given character
matches one, and only one, of each pair.
A "word" character is any letter or digit or the underscore
character, that is, any character which can be part of a
Perl "word". The definition of letters and digits is con-
trolled by PCRE's character tables, and may vary if locale-
specific matching is taking place (see "Locale support"
above). For example, in the "fr" (French) locale, some char-
acter codes greater than 128 are used for accented letters,
and these are matched by \w.
These character type sequences can appear both inside and
outside character classes. They each match one character of
the appropriate type. If the current matching point is at
the end of the subject string, all of them fail, since there
is no character to match.
The fourth use of backslash is for certain simple asser-
tions. An assertion specifies a condition that has to be met
at a particular point in a match, without consuming any
characters from the subject string. The use of subpatterns
for more complicated assertions is described below. The
backslashed assertions are
\b word boundary
\B not a word boundary
\A start of subject (independent of multiline mode)
\Z end of subject or newline at end (independent of
multiline mode)
\z end of subject (independent of multiline mode)
These assertions may not appear in character classes (but
note that "\b" has a different meaning, namely the backspace
character, inside a character class).
A word boundary is a position in the subject string where
the current character and the previous character do not both
match \w or \W (i.e. one matches \w and the other matches
\W), or the start or end of the string if the first or last
character matches \w, respectively.
The \A, \Z, and \z assertions differ from the traditional
circumflex and dollar (described below) in that they only
ever match at the very start and end of the subject string,
whatever options are set. They are not affected by the
PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu-
ment of pcre_exec() is non-zero, \A can never match. The
difference between \Z and \z is that \Z matches before a
newline that is the last character of the string as well as
at the end of the string, whereas \z matches only at the
end.
CIRCUMFLEX AND DOLLAR
Outside a character class, in the default matching mode, the
circumflex character is an assertion which is true only if
the current matching point is at the start of the subject
string. If the startoffset argument of pcre_exec() is non-
zero, circumflex can never match. Inside a character class,
circumflex has an entirely different meaning (see below).
Circumflex need not be the first character of the pattern if
a number of alternatives are involved, but it should be the
first thing in each alternative in which it appears if the
pattern is ever to match that branch. If all possible alter-
natives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is
said to be an "anchored" pattern. (There are also other con-
structs that can cause a pattern to be anchored.)
A dollar character is an assertion which is true only if the
current matching point is at the end of the subject string,
or immediately before a newline character that is the last
character in the string (by default). Dollar need not be the
last character of the pattern if a number of alternatives
are involved, but it should be the last item in any branch
in which it appears. Dollar has no special meaning in a
character class.
The meaning of dollar can be changed so that it matches only
at the very end of the string, by setting the
PCRE_DOLLAR_ENDONLY option at compile or matching time. This
does not affect the \Z assertion.
The meanings of the circumflex and dollar characters are
changed if the PCRE_MULTILINE option is set. When this is
the case, they match immediately after and immediately
before an internal "\n" character, respectively, in addition
to matching at the start and end of the subject string. For
example, the pattern /^abc$/ matches the subject string
"def\nabc" in multiline mode, but not otherwise. Conse-
quently, patterns that are anchored in single line mode
because all branches start with "^" are not anchored in mul-
tiline mode, and a match for circumflex is possible when the
startoffset argument of pcre_exec() is non-zero. The
PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
set.
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