| =head1 NAME |
| |
| perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes |
| |
| =head1 DESCRIPTION |
| |
| The top level documentation about Perl regular expressions |
| is found in L<perlre>. |
| |
| This document describes all backslash and escape sequences. After |
| explaining the role of the backslash, it lists all the sequences that have |
| a special meaning in Perl regular expressions (in alphabetical order), |
| then describes each of them. |
| |
| Most sequences are described in detail in different documents; the primary |
| purpose of this document is to have a quick reference guide describing all |
| backslash and escape sequences. |
| |
| =head2 The backslash |
| |
| In a regular expression, the backslash can perform one of two tasks: |
| it either takes away the special meaning of the character following it |
| (for instance, C<\|> matches a vertical bar, it's not an alternation), |
| or it is the start of a backslash or escape sequence. |
| |
| The rules determining what it is are quite simple: if the character |
| following the backslash is an ASCII punctuation (non-word) character (that is, |
| anything that is not a letter, digit, or underscore), then the backslash just |
| takes away any special meaning of the character following it. |
| |
| If the character following the backslash is an ASCII letter or an ASCII digit, |
| then the sequence may be special; if so, it's listed below. A few letters have |
| not been used yet, so escaping them with a backslash doesn't change them to be |
| special. A future version of Perl may assign a special meaning to them, so if |
| you have warnings turned on, Perl issues a warning if you use such a |
| sequence. [1]. |
| |
| It is however guaranteed that backslash or escape sequences never have a |
| punctuation character following the backslash, not now, and not in a future |
| version of Perl 5. So it is safe to put a backslash in front of a non-word |
| character. |
| |
| Note that the backslash itself is special; if you want to match a backslash, |
| you have to escape the backslash with a backslash: C</\\/> matches a single |
| backslash. |
| |
| =over 4 |
| |
| =item [1] |
| |
| There is one exception. If you use an alphanumeric character as the |
| delimiter of your pattern (which you probably shouldn't do for readability |
| reasons), you have to escape the delimiter if you want to match |
| it. Perl won't warn then. See also L<perlop/Gory details of parsing |
| quoted constructs>. |
| |
| =back |
| |
| |
| =head2 All the sequences and escapes |
| |
| Those not usable within a bracketed character class (like C<[\da-z]>) are marked |
| as C<Not in [].> |
| |
| \000 Octal escape sequence. See also \o{}. |
| \1 Absolute backreference. Not in []. |
| \a Alarm or bell. |
| \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 |
| \C Single octet, even under UTF-8. Not in []. |
| \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. |
| \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. |
| \k{}, \k<>, \k'' Named backreference. Not in []. |
| \K Keep the stuff left of \K. Not in []. |
| \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{} 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 Quote (disable) pattern metacharacters till \E. Not |
| in []. |
| \r Return character. |
| \R Generic new line. Not in []. |
| \s Character class for whitespace. |
| \S Character class for non whitespace. |
| \t Tab character. |
| \u Titlecase next character. Not in []. |
| \U Uppercase till \E. Not in []. |
| \v Character class for vertical whitespace. |
| \V Character class for non vertical whitespace. |
| \w Character class for word characters. |
| \W Character class for non-word characters. |
| \x{}, \x00 Hexadecimal escape sequence. |
| \X Unicode "extended grapheme cluster". Not in []. |
| \z End of string. Not in []. |
| \Z End of string. Not in []. |
| |
| =head2 Character Escapes |
| |
| =head3 Fixed characters |
| |
| A handful of characters have a dedicated I<character escape>. The following |
| table shows them, along with their ASCII code points (in decimal and hex), |
| their ASCII name, the control escape on ASCII platforms and a short |
| description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.) |
| |
| Seq. Code Point ASCII Cntrl Description. |
| Dec Hex |
| \a 7 07 BEL \cG alarm or bell |
| \b 8 08 BS \cH backspace [1] |
| \e 27 1B ESC \c[ escape character |
| \f 12 0C FF \cL form feed |
| \n 10 0A LF \cJ line feed [2] |
| \r 13 0D CR \cM carriage return |
| \t 9 09 TAB \cI tab |
| |
| =over 4 |
| |
| =item [1] |
| |
| C<\b> is the backspace character only inside a character class. Outside a |
| character class, C<\b> is a word/non-word boundary. |
| |
| =item [2] |
| |
| C<\n> matches a logical newline. Perl converts between C<\n> and your |
| OS's native newline character when reading from or writing to text files. |
| |
| =back |
| |
| =head4 Example |
| |
| $str =~ /\t/; # Matches if $str contains a (horizontal) tab. |
| |
| =head3 Control characters |
| |
| C<\c> is used to denote a control character; the character following C<\c> |
| determines the value of the construct. For example the value of C<\cA> is |
| C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc. |
| The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete |
| list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in |
| L<perlebcdic/OPERATOR DIFFERENCES>. |
| |
| Note that C<\c\> alone at the end of a regular expression (or doubled-quoted |
| string) is not valid. The backslash must be followed by another character. |
| That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>. |
| |
| To write platform-independent code, you must use C<\N{I<NAME>}> instead, like |
| C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>. |
| |
| Mnemonic: I<c>ontrol character. |
| |
| =head4 Example |
| |
| $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K). |
| |
| =head3 Named or numbered characters and character sequences |
| |
| 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. |
| |
| 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 |
| 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 |
| "A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41). |
| |
| It is even possible to give your own names to characters and character |
| sequences. For details, see L<charnames>. |
| |
| (There is an expanded internal form that you may see in debug output: |
| C<\N{U+I<code point>.I<code point>...}>. |
| The C<...> means any number of these I<code point>s separated by dots. |
| This represents the sequence formed by the characters. This is an internal |
| form only, subject to change, and you should not try to use it yourself.) |
| |
| Mnemonic: I<N>amed character. |
| |
| Note that a character or character sequence expressed as a named |
| or numbered character is considered a character without special |
| meaning by the regex engine, and will match "as is". |
| |
| =head4 Example |
| |
| $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character |
| |
| use charnames 'Cyrillic'; # Loads Cyrillic names. |
| $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA". |
| |
| =head3 Octal escapes |
| |
| There are two forms of octal escapes. Each is used to specify a character by |
| 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. |
| |
| It was introduced to avoid the potential problems with the other form, |
| available in all Perls. That form consists of a backslash followed by three |
| octal digits. One problem with this form is that it can look exactly like an |
| 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 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 |
| bugs, sometimes with surprising results. Also, if you are creating a regex |
| out of smaller snippets concatenated together, and you use fewer than three |
| digits, the beginning of one snippet may be interpreted as adding digits to the |
| ending of the snippet before it. See L</Absolute referencing> for more |
| discussion and examples of the snippet problem. |
| |
| Note that a character expressed as an octal escape is considered |
| a character without special meaning by the regex engine, and will match |
| "as is". |
| |
| To summarize, the C<\o{}> form is always safe to use, and the other form is |
| 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. |
| |
| =head4 Examples (assuming an ASCII platform) |
| |
| $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 =~ /\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) |
| |
| =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> |
| 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: |
| |
| =over 4 |
| |
| =item 1 |
| |
| If the backslash is followed by a single digit, it's a backreference. |
| |
| =item 2 |
| |
| If the first digit following the backslash is a 0, it's an octal escape. |
| |
| =item 3 |
| |
| If the number following the backslash is N (in decimal), and Perl already |
| has seen N capture groups, Perl considers this a backreference. Otherwise, |
| it considers it an octal escape. If N has more than three digits, Perl |
| takes only the first three for the octal escape; the rest are matched as is. |
| |
| my $pat = "(" x 999; |
| $pat .= "a"; |
| $pat .= ")" x 999; |
| /^($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' |
| |
| =back |
| |
| 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". |
| |
| =head3 Hexadecimal escapes |
| |
| Like octal escapes, there are two forms of hexadecimal escapes, but both start |
| with the same thing, C<\x>. This is followed by either exactly two hexadecimal |
| digits forming a number, or a hexadecimal number of arbitrary length surrounded |
| by curly braces. The hexadecimal number is the code point of the character you |
| want to express. |
| |
| Note that a character expressed as one of these escapes is considered a |
| character without special meaning by the regex engine, and will match |
| "as is". |
| |
| Mnemonic: heI<x>adecimal. |
| |
| =head4 Examples (assuming an ASCII platform) |
| |
| $str = "Perl"; |
| $str =~ /\x50/; # Match, "\x50" is "P". |
| $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once |
| $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally. |
| |
| /\x{2603}\x{2602}/ # Snowman with an umbrella. |
| # The Unicode character 2603 is a snowman, |
| # the Unicode character 2602 is an umbrella. |
| /\x{263B}/ # Black smiling face. |
| /\x{263b}/ # Same, the hex digits A - F are case insensitive. |
| |
| =head2 Modifiers |
| |
| A number of backslash sequences have to do with changing the character, |
| or characters following them. C<\l> will lowercase the character following |
| it, while C<\u> will uppercase (or, more accurately, titlecase) the |
| character following it. They provide functionality similar to the |
| functions C<lcfirst> and C<ucfirst>. |
| |
| 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>, whichever comes first. They provide functionality similar to what |
| the functions C<lc> and C<uc> provide. |
| |
| 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. |
| |
| 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 |
| |
| $sid = "sid"; |
| $greg = "GrEg"; |
| $miranda = "(Miranda)"; |
| $str =~ /\u$sid/; # Matches 'Sid' |
| $str =~ /\L$greg/; # Matches 'greg' |
| $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern |
| # had been written as /\(Miranda\)/ |
| |
| =head2 Character classes |
| |
| Perl regular expressions have a large range of character classes. Some of |
| the character classes are written as a backslash sequence. We will briefly |
| discuss those here; full details of character classes can be found in |
| L<perlrecharclass>. |
| |
| C<\w> is a character class that matches any single I<word> character |
| (letters, digits, Unicode marks, and connector punctuation (like the |
| underscore)). C<\d> is a character class that matches any decimal |
| digit, while the character class C<\s> matches any whitespace character. |
| New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal |
| 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. 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 |
| word character, digit, whitespace, horizontal whitespace, or vertical |
| whitespace. |
| |
| Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical. |
| |
| =head3 Unicode classes |
| |
| C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to |
| match a character that matches the given Unicode property; properties |
| include things like "letter", or "thai character". Capitalizing the |
| sequence to C<\PP> and C<\P{Property}> make the sequence match a character |
| that doesn't match the given Unicode property. For more details, see |
| L<perlrecharclass/Backslash sequences> and |
| L<perlunicode/Unicode Character Properties>. |
| |
| Mnemonic: I<p>roperty. |
| |
| =head2 Referencing |
| |
| If capturing parenthesis are used in a regular expression, we can refer |
| to the part of the source string that was matched, and match exactly the |
| same thing. There are three ways of referring to such I<backreference>: |
| absolutely, relatively, and by name. |
| |
| =for later add link to perlrecapture |
| |
| =head3 Absolute referencing |
| |
| Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N> |
| is a positive (unsigned) decimal number of any length is an absolute reference |
| to a capturing group. |
| |
| I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has |
| been matched by that set of parentheses. Thus C<\g1> refers to the first |
| capture group in the regex. |
| |
| The C<\gI<N>> form can be equivalently written as C<\g{I<N>}> |
| which avoids ambiguity when building a regex by concatenating shorter |
| strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained |
| C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is |
| probably not what you intended. |
| |
| In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at |
| least I<N> capturing groups, or else I<N> is considered an octal escape |
| (but something like C<\18> is the same as C<\0018>; that is, the octal escape |
| C<"\001"> followed by a literal digit C<"8">). |
| |
| Mnemonic: I<g>roup. |
| |
| =head4 Examples |
| |
| /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat"). |
| /(\w+) \1/; # Same thing; written old-style |
| /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA"). |
| |
| |
| =head3 Relative referencing |
| |
| C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can |
| be written as C<\g{-I<N>>.) It refers to the I<N>th group before the |
| C<\g{-I<N>}>. |
| |
| The big advantage of this form is that it makes it much easier to write |
| patterns with references that can be interpolated in larger patterns, |
| even if the larger pattern also contains capture groups. |
| |
| =head4 Examples |
| |
| /(A) # Group 1 |
| ( # Group 2 |
| (B) # Group 3 |
| \g{-1} # Refers to group 3 (B) |
| \g{-3} # Refers to group 1 (A) |
| ) |
| /x; # Matches "ABBA". |
| |
| my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc. |
| /$qr$qr/ # Matches 'ababcdcd'. |
| |
| =head3 Named referencing |
| |
| C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a |
| named capture group, dispensing completely with having to think about capture |
| buffer positions. |
| |
| To be compatible with .Net regular expressions, C<\g{name}> may also be |
| written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>. |
| |
| To prevent any ambiguity, I<name> must not start with a digit nor contain a |
| hyphen. |
| |
| =head4 Examples |
| |
| /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat") |
| /(?<word>\w+) \k{word}/ # Same. |
| /(?<word>\w+) \k<word>/ # Same. |
| /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/ |
| # Match a four letter palindrome (e.g. "ABBA") |
| |
| =head2 Assertions |
| |
| Assertions are conditions that have to be true; they don't actually |
| match parts of the substring. There are six assertions that are written as |
| backslash sequences. |
| |
| =over 4 |
| |
| =item \A |
| |
| C<\A> only matches at the beginning of the string. If the C</m> modifier |
| isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m> |
| modifier is used, then C</^/> matches internal newlines, but the meaning |
| of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning |
| of the string regardless whether the C</m> modifier is used. |
| |
| =item \z, \Z |
| |
| C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't |
| used, then C</\Z/> is equivalent to C</$/>; that is, it matches at the |
| end of the string, or one before the newline at the end of the string. If the |
| C</m> modifier is used, then C</$/> matches at internal newlines, but the |
| meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at |
| the end of the string (or just before a trailing newline) regardless whether |
| the C</m> modifier is used. |
| |
| C<\z> is just like C<\Z>, except that it does not match before a trailing |
| newline. C<\z> matches at the end of the string only, regardless of the |
| modifiers used, and not just before a newline. It is how to anchor the |
| match to the true end of the string under all conditions. |
| |
| =item \G |
| |
| C<\G> is usually used only in combination with the C</g> modifier. If the |
| C</g> modifier is used and the match is done in scalar context, Perl |
| remembers where in the source string the last match ended, and the next time, |
| it will start the match from where it ended the previous time. |
| |
| C<\G> matches the point where the previous match on that string ended, |
| or the beginning of that string if there was no previous match. |
| |
| =for later add link to perlremodifiers |
| |
| Mnemonic: I<G>lobal. |
| |
| =item \b, \B |
| |
| C<\b> matches at any place between a word and a non-word character; C<\B> |
| matches at any place between characters where C<\b> doesn't match. C<\b> |
| and C<\B> assume there's a non-word character before the beginning and after |
| the end of the source string; so C<\b> will match at the beginning (or end) |
| of the source string if the source string begins (or ends) with a word |
| character. Otherwise, C<\B> will match. |
| |
| Do not use something like C<\b=head\d\b> and expect it to match the |
| beginning of a line. It can't, because for there to be a boundary before |
| the non-word "=", there must be a word character immediately previous. |
| All boundary determinations look for word characters alone, not for |
| non-words characters nor for string ends. It may help to understand how |
| <\b> and <\B> work by equating them as follows: |
| |
| \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w)) |
| \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w)) |
| |
| Mnemonic: I<b>oundary. |
| |
| =back |
| |
| =head4 Examples |
| |
| "cat" =~ /\Acat/; # Match. |
| "cat" =~ /cat\Z/; # Match. |
| "cat\n" =~ /cat\Z/; # Match. |
| "cat\n" =~ /cat\z/; # No match. |
| |
| "cat" =~ /\bcat\b/; # Matches. |
| "cats" =~ /\bcat\b/; # No match. |
| "cat" =~ /\bcat\B/; # No match. |
| "cats" =~ /\bcat\B/; # Match. |
| |
| while ("cat dog" =~ /(\w+)/g) { |
| print $1; # Prints 'catdog' |
| } |
| while ("cat dog" =~ /\G(\w+)/g) { |
| print $1; # Prints 'cat' |
| } |
| |
| =head2 Misc |
| |
| Here we document the backslash sequences that don't fall in one of the |
| categories above. These are: |
| |
| =over 4 |
| |
| =item \C |
| |
| C<\C> always matches a single octet, even if the source string is encoded |
| in UTF-8 format, and the character to be matched is a multi-octet character. |
| C<\C> was introduced in perl 5.6. This is very dangerous, because it violates |
| the logical character abstraction and can cause UTF-8 sequences to become malformed. |
| |
| Mnemonic: oI<C>tet. |
| |
| =item \K |
| |
| This appeared in perl 5.10.0. Anything matched left of C<\K> is |
| not included in C<$&>, and will not be replaced if the pattern is |
| used in a substitution. This lets you write C<s/PAT1 \K PAT2/REPL/x> |
| instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>. |
| |
| Mnemonic: I<K>eep. |
| |
| =item \N |
| |
| This is an experimental feature new to perl 5.12.0. It matches any character |
| that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is |
| identical to the C<.> metasymbol, except under the C</s> flag, which changes |
| the meaning of C<.>, but not C<\N>. |
| |
| Note that C<\N{...}> can mean a |
| L<named or numbered character |
| |/Named or numbered characters and character sequences>. |
| |
| Mnemonic: Complement of I<\n>. |
| |
| =item \R |
| X<\R> |
| |
| C<\R> matches a I<generic newline>; that is, anything considered a |
| linebreak sequence by Unicode. This includes all characters matched by |
| C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A"> |
| (carriage return followed by a line feed, sometimes called the network |
| newline; it's the end of line sequence used in Microsoft text files opened |
| in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. (The |
| reason it doesn't backtrack is that the sequence is considered |
| inseparable. That means that |
| |
| "\x0D\x0A" =~ /^\R\x0A$/ # 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. |
| |
| =item \X |
| X<\X> |
| |
| This matches a Unicode I<extended grapheme cluster>. |
| |
| C<\X> matches quite well what normal (non-Unicode-programmer) usage |
| would consider a single character. As an example, consider a G with some sort |
| of diacritic mark, such as an arrow. There is no such single character in |
| Unicode, but one can be composed by using a G followed by a Unicode "COMBINING |
| UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it |
| were a single character. |
| |
| 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' |
| $str =~ s/(.)\K\g1//g; # Delete duplicated characters. |
| |
| "\n" =~ /^\R$/; # Match, \n is a generic newline. |
| "\r" =~ /^\R$/; # Match, \r is a generic newline. |
| "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline. |
| |
| "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above. |
| |
| =cut |