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NAME

       perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes

DESCRIPTION

       The top level documentation about Perl regular expressions is found in 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.

   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, "\|" 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: "/\\/" matches a single backslash.

       [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 "Gory details of parsing quoted constructs" in perlop.

   All the sequences and escapes
       Those not usable within a bracketed character class (like "[\da-z]") are marked as "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{}, \b          Boundary. (\b is a backspace in []).
        \B{}, \B          Not a boundary.  Not in [].
        \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.
        \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.  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 [].

   Character Escapes
       Fixed characters

       A handful of characters have a dedicated 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 "OPERATOR DIFFERENCES" in perlebcdic.)

        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

       [1] "\b" is the backspace character only inside a character class. Outside a character class, "\b" alone
           is a word-character/non-word-character boundary, and "\b{}" is some other type of boundary.

       [2] "\n" matches a logical newline. Perl converts between "\n" and your OS's native newline character
           when reading from or writing to text files.

       Example

        $str =~ /\t/;   # Matches if $str contains a (horizontal) tab.

       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 sequence "\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.

   Modifiers
       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{}, \B
           "\b{...}", available starting in v5.22, matches a boundary (between two characters, or before the
           first character of the string, or after the final character of the string) based on the Unicode rules
           for the boundary type specified inside the braces.  The currently known boundary types are given a
           few paragraphs below.  "\B{...}" matches at any place between characters where "\b{...}" of the same
           type doesn't match.

           "\b" when not immediately followed by a "{" matches at any place between a word (something matched by
           "\w") and a non-word character ("\W"); "\B" when not immediately followed by a "{" matches at any
           place between characters where "\b" doesn't match.  To get better word matching of natural language
           text, see \b{wb} below.

           "\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 plain "\b" and "\B" boundary determinations look for word characters
           alone, not for non-word 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))

           In contrast, "\b{...}" and "\B{...}" may or may not match at the beginning and end of the line,
           depending on the boundary type.  These implement the Unicode default boundaries, specified in
           <http://www.unicode.org/reports/tr29/>.  The boundary types currently available are:

           "\b{gcb}" or "\b{g}"
               This matches a Unicode "Grapheme Cluster Boundary".  (Actually Perl always uses the improved
               "extended" grapheme cluster").  These are explained below under ""\X"".  In fact, "\X" is another
               way to get the same functionality.  It is equivalent to "/.+?\b{gcb}/".  Use whichever is most
               convenient for your situation.

           "\b{sb}"
               This matches a Unicode "Sentence Boundary".  This is an aid to parsing natural language
               sentences.  It gives good, but imperfect results.  For example, it thinks that "Mr. Smith" is two
               sentences.  More details are at <http://www.unicode.org/reports/tr29/>.  Note also that it thinks
               that anything matching "\R" (except form feed and vertical tab) is a sentence boundary.  "\b{sb}"
               works with text designed for word-processors which wrap lines automatically for display, but
               hard-coded line boundaries are considered to be essentially the ends of text blocks (paragraphs
               really), and hence the ends of sententces.  "\b{sb}" doesn't do well with text containing
               embedded newlines, like the source text of the document you are reading.  Such text needs to be
               preprocessed to get rid of the line separators before looking for sentence boundaries.  Some
               people view this as a bug in the Unicode standard, and this behavior is quite subject to change
               in future Perl versions.

           "\b{wb}"
               This matches a Unicode "Word Boundary".  This gives better (though not perfect) results for
               natural language processing than plain "\b" (without braces) does.  For example, it understands
               that apostrophes can be in the middle of words and that parentheses aren't (see the examples
               below).   More details are at <http://www.unicode.org/reports/tr29/>.

           It is important to realize when you use these Unicode boundaries, that you are taking a risk that a
           future version of Perl which contains a later version of the Unicode Standard will not work precisely
           the same way as it did when your code was written.  These rules are not considered stable and have
           been somewhat more subject to change than the rest of the Standard.  Unicode reserves the right to
           change them at will, and Perl reserves the right to update its implementation to Unicode's new rules.
           In the past, some changes have been because new characters have been added to the Standard which have
           different characteristics than all previous characters, so new rules are formulated for handling
           them.  These should not cause any backward compatibility issues.  But some changes have changed the
           treatment of existing characters because the Unicode Technical Committee has decided that the change
           is warranted for whatever reason.  This could be to fix a bug, or because they think better results
           are obtained with the new rule.

           It is also important to realize that these are default boundary definitions, and that implementations
           may wish to tailor the results for particular purposes and locales.

           Unicode defines a fourth boundary type, accessible through the Unicode::LineBreak module.

           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'
         }

         my $s = "He said, \"Is pi 3.14? (I'm not sure).\"";
         print join("|", $s =~ m/ ( .+? \b     ) /xg), "\n";
         print join("|", $s =~ m/ ( .+? \b{wb} ) /xg), "\n";
        prints
         He| |said|, "|Is| |pi| |3|.|14|? (|I|'|m| |not| |sure
         He| |said|,| |"|Is| |pi| |3.14|?| |(|I'm| |not| |sure|)|.|"

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

       \C  (Deprecated.) "\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.

           Use "utf8::encode()" instead.

           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.

           The match is greedy and non-backtracking, so that the cluster is never broken up into smaller
           components.

           See also "\b{gcb}".

           Mnemonic: eXtended Unicode character.

       Examples

        $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.