НАИМЕНОВАНИЕ

perlrebackslash - Бэкслеш и Эскейп последовательности в регулярных выражениях Perl

ОПИСАНИЕ

Документация верхнего уровня о регулярных выражениях Perl находится в perlre.

Этот документ описывает все backslash и escape последовательности. После объяснения роли обратной косой черты, перечисляются все последовательности, которые имеют специальное значение в регулярных выражениях Perl (в алфавитном порядке), затем описывается каждая из них.

Большинство последовательностей описаны в деталях в различных документах; основная цель этого документа заключается в кратком справочном руководстве, описывающем все backslash и escape последовательности.

Обратная косая черта (backslash)

В регулярном выражении обратной косой чертой можно выполнить одну из двух задач: Она либо отнимает особое значение символа после него (например, \| найдет вертикальную черту, здесь это не знак или), или это начало последовательности, обратной косой черты или эскейп (backslash или escape sequence).

Правила, определяющие, что это, довольно просты: eсли символ после обратной косой черты является знаком ASCII пунктуации (не слово) (то есть, все, что не является буквой, цифрой или подчеркивания), то обратная косая черта просто снимает любое особое значение символа, после него.

Если символ после обратной косой черты ASCII буква или ASCII цифра, тогда последовательность может быть специальной; если это так, то она приводится ниже. Есть несколько букв, которые не используются, так что их эскейпинг косой чертой не меняет их специальности. Будущие версии Perl может назначить особый смысл для них, так что если у вас есть включенные предупреждения (use warnings), Perl выдает предупреждение, если вы используете такую последовательность. [1].

Однако, гарантируется, что последовательности обратной косой черты или эскейп никогда не будут иметь знака препинания после обратной косой черты, ни сейчас, ни в будущих версиях Perl 5. Так что это безопасно поставить косую черту перед несловесным символом.

Обратите внимание, что обратная косая черта является специальным символом; если вы хотите найти обратную косую черту, вы должны заэскейпить ее обратной косой чертой:/\\/ найдет одну косую черту (backslash).

[1]

Есть одно исключение. Если вы используете алфавитно-цифровой символ как разделитель вашего шаблона (который вы, вероятно, не стоит делать по причине удобочитаемости), вы должны заэскейпить разделитель, если вы хотите найти его. Тогда Perl не будет предупреждать. Смотрите также "Подробности разбора (парсинга) конструкций в кавычках" in perlop.

Все последовательности и эскейпы

Тех, что не используются в пределах класса символов в квадратных скобосках (таких как [\da-z]) помечаются как Не в [].

\000              Восьмеричная escape-последовательность.  Смотри также \o{}.
\1                Абсолютная обратная ссылка.  Не в [].
\a                Будильник или Звонок.
\A                Начало строки.  Не в [].
\b                Граница слова/не-слова. (Обратный слеш в []).
\B                Не граница слова/не-слова. Не в [].
\cX               Control-X.
\C                Один октет, даже под UTF-8.  Не в [].
\d                Класс символов для цифр.
\D                Класс символов для не-цифр.
\e                Символ эскейпа.
\E                Выключает обоработку \Q, \L и \U. Не в [].
\f                Перевод формата (Formfeed).
\F                Возвращает регистронезависимое представление (Foldcase) до \E.  Не в [].
\g{}, \g1         Именованная, абсолютная или относительная обратная ссылка. Не в [].
\G                Найдет, где остановился предыдущий m//g Позиция поиска в регекспе (Pos).  Не в [].
\h                Класс символов для горизонтальных пробелов.
\H                Класс символов для не-горизонтальных пробелов.
\k{}, \k<>, \k''  Именованная обратная ссылка. Не в [].
\K                Сохраняет найденный материал слева от \K, не включая его в $&. Не в [].
\l                Следующий символ в нижнем регистре.  Не в [].
\L                Нижний регистр до \E.  Не в [].
\n                (Логически) символ новой строки.
\N                Любой символ, кроме символа новой строки.  Не в [].
\N{}              Именованный или номерной символ (Юникода) или последовательности.
\o{}              Восьмеричная эскейп-последовательность.
\p{}, \pP         Символ с данным Юникодным свойством.
\P{}, \PP         Символ без данного Юникодного свойства.
\Q                Квотирует (отключает) действие метасиволов в шаблоне до \E.  Не в [].
\r                Символ возврата каретки.
\R                Универсальная новая строка           (?>\v|\x0D\x0A)  Не в [].
\s                Класс символов для пробелов.
\S                Класс символов для не-пробелов.
\t                Символ табуляции.
\u                Следующий символ в верхнем регистре.  Не в [].
\U                Верхний регистр до \E.  Не в [].
\v                Класс символов для вертикальных пробелов.
\V                Класс символов для не-вертикальных пробелов.
\w                Класс символов для слова.
\W                Класс символов для не-слова.
\x{}, \x00        Шестнадцатеричная эскейп-последовательность.
\X                Найдет Юникодный расширенный графема кластер ("extended grapheme cluster").  Не в [].
\z                Конец строки.  Не в [].
\Z                Конец строки.  Не в [].

Эскейп символы

Фиксированные знаки

У горстки знаков есть специальные эскейп символы. Следующая таблица показывает их, наряду с их кодами ASCII (в десятичном и шестнадцатиричном виде), их ASCII имя , контрол эскейп на ASCII платформах и короткое описание. (Для платформ расширенного двоично-десятичного кода (EBCDIC) см. "ОПЕРАТОР РАЗЛИЧИЯ" in perlebcdic.)

Seq.  Code Point  ASCII   Cntrl   Description.
      Dec    Hex
 \a     7     07    BEL    \cG    будильник или звонок.
 \b     8     08     BS    \cH    обратный слеш [1]
 \e    27     1B    ESC    \c[    символ эскейпа
 \f    12     0C     FF    \cL    перевод формата
 \n    10     0A     LF    \cJ    перевод строки [2]
 \r    13     0D     CR    \cM    возврат каретки
 \t     9     09    TAB    \cI    табуляция

[1]

\b это бекслешированный символ действует только внутри класса символов. Вне класс символов C <\b> это граница слово/не-слова.

[2]

\n соответствует логической новой строке. Perl преобразует между \n и ваш ОС родной символ новой строки при чтении или записи в текстовые файлы.

Пример

$str =~ /\t/;   # Найдет, если $str содержит (горизонтальный) символ табуляции.

Control characters

\c is used to denote a control character; the character following \c determines the value of the construct. For example the value of \cA is chr(1), and the value of \cb is chr(2), etc. The gory details are in "Regexp Quote-Like Operators" in perlop. A complete list of what chr(1), etc. means for ASCII and EBCDIC platforms is in "OPERATOR DIFFERENCES" in perlebcdic.

Note that \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\X means chr(28) . 'X' for all characters X.

To write platform-independent code, you must use \N{NAME} instead, like \N{ESCAPE} or \N{U+001B}, see charnames.

Mnemonic: control character.

Example

$str =~ /\cK/;  # Matches if $str contains a vertical tab (control-K).

Named or numbered characters and character sequences

Unicode characters have a Unicode name and numeric code point (ordinal) value. Use the \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 \N{U+code point}, where 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 \N{U+0041} means LATIN CAPITAL LETTER A, and you will rarely see it written without the two leading zeros. \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 charnames.

(There is an expanded internal form that you may see in debug output: \N{U+code point.code point...}. The ... means any number of these code points 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: Named 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".

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".

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 \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 "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 "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 \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: 0ctal or octal.

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).

Disambiguation rules between old-style octal escapes and backreferences

Octal escapes of the \000 form outside of bracketed character classes potentially clash with old-style backreferences (see "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:

1. If the backslash is followed by a single digit, it's a backreference.

2. If the first digit following the backslash is a 0, it's an octal escape.

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'.

You can force a backreference interpretation always by using the \g{...} form. You can the force an octal interpretation always by using the \o{...} form, or for numbers up through \077 (= 63 decimal), by using three digits, beginning with a "0".

Hexadecimal escapes

Like octal escapes, there are two forms of hexadecimal escapes, but both start with the same thing, \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: hexadecimal.

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.

Модификаторы

A number of backslash sequences have to do with changing the character, or characters following them. \l will lowercase the character following it, while \u will uppercase (or, more accurately, titlecase) the character following it. They provide functionality similar to the functions lcfirst and ucfirst.

To uppercase or lowercase several characters, one might want to use \L or \U, which will lowercase/uppercase all characters following them, until either the end of the pattern or the next occurrence of \E, whichever comes first. They provide functionality similar to what the functions lc and uc provide.

\Q is used to quote (disable) pattern metacharacters, up to the next \E or the end of the pattern. \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 "quotemeta" in perlfunc for details on what gets quoted for non-ASCII code points. Using this ensures that any character between \Q and \E will be matched literally, not interpreted as a metacharacter by the regex engine.

\F can be used to casefold all characters following, up to the next \E or the end of the pattern. It provides the functionality similar to the fc function.

Mnemonic: Lowercase, Uppercase, Fold-case, Quotemeta, End.

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 /\(Miranda\)/

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 perlrecharclass.

\w is a character class that matches any single word character (letters, digits, Unicode marks, and connector punctuation (like the underscore)). \d is a character class that matches any decimal digit, while the character class \s matches any whitespace character. New in perl 5.10.0 are the classes \h and \v which match horizontal and vertical whitespace characters.

The exact set of characters matched by \d, \s, and \w varies depending on various pragma and regular expression modifiers. It is possible to restrict the match to the ASCII range by using the /a regular expression modifier. See perlrecharclass.

The uppercase variants (\W, \D, \S, \H, and \V) are character classes that match, respectively, any character that isn't a word character, digit, whitespace, horizontal whitespace, or vertical whitespace.

Mnemonics: word, digit, space, horizontal, vertical.

Unicode classes

\pP (where P is a single letter) and \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 \PP and \P{Property} make the sequence match a character that doesn't match the given Unicode property. For more details, see "Backslash sequences" in perlrecharclass and "Unicode Character Properties" in perlunicode.

Mnemonic: property.

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 backreference: absolutely, relatively, and by name.

Absolute referencing

Either \gN (starting in Perl 5.10.0), or \N (old-style) where N is a positive (unsigned) decimal number of any length is an absolute reference to a capturing group.

N refers to the Nth set of parentheses, so \gN refers to whatever has been matched by that set of parentheses. Thus \g1 refers to the first capture group in the regex.

The \gN form can be equivalently written as \g{N} which avoids ambiguity when building a regex by concatenating shorter strings. Otherwise if you had a regex qr/$a$b/, and $a contained "\g1", and $b contained "37", you would get /\g137/ which is probably not what you intended.

In the \N form, N must not begin with a "0", and there must be at least N capturing groups, or else N is considered an octal escape (but something like \18 is the same as \0018; that is, the octal escape "\001" followed by a literal digit "8").

Mnemonic: group.

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").

Relative referencing

\g-N (starting in Perl 5.10.0) is used for relative addressing. (It can be written as \g{-N.) It refers to the Nth group before the \g{-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.

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'.

Named referencing

\g{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, \g{name} may also be written as \k{name}, \k<name> or \k'name'.

To prevent any ambiguity, name must not start with a digit nor contain a hyphen.

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")

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.

\A

\A only matches at the beginning of the string. If the /m modifier isn't used, then /\A/ is equivalent to /^/. However, if the /m modifier is used, then /^/ matches internal newlines, but the meaning of /\A/ isn't changed by the /m modifier. \A matches at the beginning of the string regardless whether the /m modifier is used.

\z, \Z

\z and \Z match at the end of the string. If the /m modifier isn't used, then /\Z/ is equivalent to /$/; that is, it matches at the end of the string, or one before the newline at the end of the string. If the /m modifier is used, then /$/ matches at internal newlines, but the meaning of /\Z/ isn't changed by the /m modifier. \Z matches at the end of the string (or just before a trailing newline) regardless whether the /m modifier is used.

\z is just like \Z, except that it does not match before a trailing newline. \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.

\G

\G is usually used only in combination with the /g modifier. If the /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.

\G matches the point where the previous match on that string ended, or the beginning of that string if there was no previous match.

Mnemonic: Global.

\b, \B

\b matches at any place between a word and a non-word character; \B matches at any place between characters where \b doesn't match. \b and \B assume there's a non-word character before the beginning and after the end of the source string; so \b will match at the beginning (or end) of the source string if the source string begins (or ends) with a word character. Otherwise, \B will match.

Do not use something like \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: boundary.

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'
}

Misc

Here we document the backslash sequences that don't fall in one of the categories above. These are:

\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. This is very dangerous, because it violates the logical character abstraction and can cause UTF-8 sequences to become malformed.

Mnemonic: oCtet.

\K

This appeared in perl 5.10.0. Anything matched left of \K is not included in $&, and will not be replaced if the pattern is used in a substitution. This lets you write s/PAT1 \K PAT2/REPL/x instead of s/(PAT1) PAT2/${1}REPL/x or s/(?<=PAT1) PAT2/REPL/x.

Mnemonic: Keep.

\N

This feature, available starting in v5.12, matches any character that is not a newline. It is a short-hand for writing [^\n], and is identical to the . metasymbol, except under the /s flag, which changes the meaning of ., but not \N.

Note that \N{...} can mean a named or numbered character .

Mnemonic: Complement of \n.

\R

\R matches a generic newline; that is, anything considered a linebreak sequence by Unicode. This includes all characters matched by \v (vertical whitespace), and the multi character sequence "\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). \R is equivalent to (?>\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 \R matches the entire string, and won't backtrack to match just the "\x0D".) Since \R can match a sequence of more than one character, it cannot be put inside a bracketed character class; /[\R]/ is an error; use \v instead. \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. \R was picked because PCRE already uses \R, and more importantly because Unicode recommends such a regular expression metacharacter, and suggests \R as its notation.

\X

This matches a Unicode extended grapheme cluster.

\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: eXtended Unicode character.

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.

cpanm POD2::RU

perl -MCPAN -e shell
install POD2::RU