\A Beginning of string. Not in [].
\b Word/non-word boundary. (Backspace in []).
\B Not a word/non-word boundary. Not in [].
- \cX Control-X
+ \cX Control-X.
\C Single octet, even under UTF-8. Not in [].
+ (Deprecated)
\d Character class for digits.
\D Character class for non-digits.
\e Escape character.
\E Turn off \Q, \L and \U processing. Not in [].
\f Form feed.
- \g{}, \g1 Named, absolute or relative backreference. Not in []
+ \F Foldcase till \E. Not in [].
+ \g{}, \g1 Named, absolute or relative backreference.
+ Not in [].
\G Pos assertion. Not in [].
\h Character class for horizontal whitespace.
\H Character class for non horizontal whitespace.
\l Lowercase next character. Not in [].
\L Lowercase till \E. Not in [].
\n (Logical) newline character.
- \N Any character but newline. Experimental. Not in [].
+ \N Any character but newline. Not in [].
\N{} Named or numbered (Unicode) character or sequence.
\o{} Octal escape sequence.
\p{}, \pP Character with the given Unicode property.
\P{}, \PP Character without the given Unicode property.
- \Q Quotemeta till \E. Not in [].
+ \Q Quote (disable) pattern metacharacters till \E. Not
+ in [].
\r Return character.
\R Generic new line. Not in [].
\s Character class for whitespace.
=head3 Named or numbered characters and character sequences
-Unicode characters have a Unicode name and numeric ordinal value. Use the
+Unicode characters have a Unicode name and numeric code point (ordinal)
+value. Use the
C<\N{}> construct to specify a character by either of these values.
Certain sequences of characters also have names.
To specify by name, the name of the character or character sequence goes
-between the curly braces. In this case, you have to C<use charnames> to
-load the Unicode names of the characters; otherwise Perl will complain.
+between the curly braces.
To specify a character by Unicode code point, use the form C<\N{U+I<code
point>}>, where I<code point> is a number in hexadecimal that gives the
-ordinal number that Unicode has assigned to the desired character. It is
+code point that Unicode has assigned to the desired character. It is
customary but not required to use leading zeros to pad the number to 4
digits. Thus C<\N{U+0041}> means C<LATIN CAPITAL LETTER A>, and you will
rarely see it written without the two leading zeros. C<\N{U+0041}> means
=head4 Example
- use charnames ':full'; # Loads the Unicode names.
$str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
use charnames 'Cyrillic'; # Loads Cyrillic names.
=head3 Octal escapes
There are two forms of octal escapes. Each is used to specify a character by
-its ordinal, specified in octal notation.
+its code point specified in octal notation.
One form, available starting in Perl 5.14 looks like C<\o{...}>, where the dots
represent one or more octal digits. It can be used for any Unicode character.
old-style backreference (see
L</Disambiguation rules between old-style octal escapes and backreferences>
below.) You can avoid this by making the first of the three digits always a
-zero, but that makes \077 the largest ordinal unambiguously specifiable by this
-form.
+zero, but that makes \077 the largest code point specifiable.
In some contexts, a backslash followed by two or even one octal digits may be
interpreted as an octal escape, sometimes with a warning, and because of some
"as is".
To summarize, the C<\o{}> form is always safe to use, and the other form is
-safe to use for ordinals up through \077 when you use exactly three digits to
+safe to use for code points through \077 when you use exactly three digits to
specify them.
Mnemonic: I<0>ctal or I<o>ctal.
$str = "Perl";
$str =~ /\o{120}/; # Match, "\120" is "P".
$str =~ /\120/; # Same.
- $str =~ /\o{120}+/; # Match, "\120" is "P", it's repeated at least once
+ $str =~ /\o{120}+/; # Match, "\120" is "P",
+ # it's repeated at least once.
$str =~ /\120+/; # Same.
$str =~ /P\053/; # No match, "\053" is "+" and taken literally.
/\o{23073}/ # Black foreground, white background smiling face.
- /\o{4801234567}/ # Raises a warning, and yields chr(4)
+ /\o{4801234567}/ # Raises a warning, and yields chr(4).
=head4 Disambiguation rules between old-style octal escapes and backreferences
Octal escapes of the C<\000> form outside of bracketed character classes
-potentially clash with old-style backreferences. (see L</Absolute referencing>
+potentially clash with old-style backreferences (see L</Absolute referencing>
below). They both consist of a backslash followed by numbers. So Perl has to
use heuristics to determine whether it is a backreference or an octal escape.
Perl uses the following rules to disambiguate:
$pat .= ")" x 999;
/^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
/^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
- # and \1000 is seen as \100 (a '@') and a '0'
+ # and \1000 is seen as \100 (a '@') and a '0'.
=back
-You can the force a backreference interpretation always by using the C<\g{...}>
+You can force a backreference interpretation always by using the C<\g{...}>
form. You can the force an octal interpretation always by using the C<\o{...}>
form, or for numbers up through \077 (= 63 decimal), by using three digits,
beginning with a "0".
To uppercase or lowercase several characters, one might want to use
C<\L> or C<\U>, which will lowercase/uppercase all characters following
them, until either the end of the pattern or the next occurrence of
-C<\E>, whatever comes first. They provide functionality similar to what
+C<\E>, whichever comes first. They provide functionality similar to what
the functions C<lc> and C<uc> provide.
-C<\Q> is used to escape all characters following, up to the next C<\E>
-or the end of the pattern. C<\Q> adds a backslash to any character that
-isn't a letter, digit, or underscore. This ensures that any character
-between C<\Q> and C<\E> shall be matched literally, not interpreted
-as a metacharacter by the regex engine.
+C<\Q> is used to quote (disable) pattern metacharacters, up to the next
+C<\E> or the end of the pattern. C<\Q> adds a backslash to any character
+that could have special meaning to Perl. In the ASCII range, it quotes
+every character that isn't a letter, digit, or underscore. See
+L<perlfunc/quotemeta> for details on what gets quoted for non-ASCII
+code points. Using this ensures that any character between C<\Q> and
+C<\E> will be matched literally, not interpreted as a metacharacter by
+the regex engine.
-Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
+C<\F> can be used to casefold all characters following, up to the next C<\E>
+or the end of the pattern. It provides the functionality similar to
+the C<fc> function.
+
+Mnemonic: I<L>owercase, I<U>ppercase, I<F>old-case, I<Q>uotemeta, I<E>nd.
=head4 Examples
and vertical whitespace characters.
The exact set of characters matched by C<\d>, C<\s>, and C<\w> varies
-depending on various pragma and regular expression modifiers. See
-L<perlre>.
+depending on various pragma and regular expression modifiers. It is
+possible to restrict the match to the ASCII range by using the C</a>
+regular expression modifier. See L<perlrecharclass>.
The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
character classes that match, respectively, any character that isn't a
=head4 Examples
/(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
- /(\w+) \1/; # Same thing; written old-style
+ /(\w+) \1/; # Same thing; written old-style.
/(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
=item \C
-C<\C> always matches a single octet, even if the source string is encoded
+(Deprecated.) C<\C> always matches a single octet, even if the source
+string is encoded
in UTF-8 format, and the character to be matched is a multi-octet character.
-C<\C> was introduced in perl 5.6. This is very dangerous, because it violates
+This is very dangerous, because it violates
the logical character abstraction and can cause UTF-8 sequences to become malformed.
+Use C<utf8::encode()> instead.
+
Mnemonic: oI<C>tet.
=item \K
=item \N
-This is an experimental feature new to perl 5.12.0. It matches any character
+This feature, available starting in v5.12, matches any character
that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is
identical to the C<.> metasymbol, except under the C</s> flag, which changes
the meaning of C<.>, but not C<\N>.
C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
(carriage return followed by a line feed, sometimes called the network
newline; it's the end of line sequence used in Microsoft text files opened
-in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A)|\v) >>. Since
+in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. (The
+reason it doesn't backtrack is that the sequence is considered
+inseparable. That means that
+
+ "\x0D\x0A" =~ /^\R\x0A$/ # No match
+
+fails, because the C<\R> matches the entire string, and won't backtrack
+to match just the C<"\x0D">.) Since
C<\R> can match a sequence of more than one character, it cannot be put
inside a bracketed character class; C</[\R]/> is an error; use C<\v>
instead. C<\R> was introduced in perl 5.10.0.
+Note that this does not respect any locale that might be in effect; it
+matches according to the platform's native character set.
+
Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
and more importantly because Unicode recommends such a regular expression
metacharacter, and suggests C<\R> as its notation.
UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
were a single character.
+The match is greedy and non-backtracking, so that the cluster is never
+broken up into smaller components.
+
Mnemonic: eI<X>tended Unicode character.
=back
=head4 Examples
- "\x{256}" =~ /^\C\C$/; # Match as chr (0x256) takes 2 octets in UTF-8.
-
$str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
$str =~ s/(.)\K\g1//g; # Delete duplicated characters.
https://perl5.git.perl.org / perl5.git / blobdiff