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X11R7.4 - man page for perlrebackslash (x11r4 section 1)

PERLREBACKSLASH(1)		 Perl Programmers Reference Guide	       PERLREBACKSLASH(1)

       perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes

       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 back-
       slash is a punctuation (non-word) character (that is, anything that is not a letter, digit
       or underscore), then the backslash just takes away the special meaning (if any) of the
       character following it.

       If the character following the backslash is a letter or a digit, then the sequence may be
       special; if so, it's listed below. A few letters have not been used yet, and escaping them
       with a backslash is safe for now, but a future version of Perl may assign a special mean-
       ing to it. However, if you have warnings turned on, Perl will issue a warning if you use
       such a sequence.  [1].

       It is however guaranteed that backslash or escape sequences never have a punctuation char-
       acter 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 alphanumerical character as the delimiter of
	   your pattern (which you probably shouldn't do for readability reasons), you will 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

	\000		  Octal escape sequence.
	\1		  Absolute backreference.
	\a		  Alarm or bell.
	\A		  Beginning of string.
	\b		  Word/non-word boundary. (Backspace in a char class).
	\B		  Not a word/non-word boundary.
	\cX		  Control-X (X can be any ASCII character).
	\C		  Single octet, even under UTF-8.
	\d		  Character class for digits.
	\D		  Character class for non-digits.
	\e		  Escape character.
	\E		  Turn off \Q, \L and \U processing.
	\f		  Form feed.
	\G		  Pos assertion.
	\l		  Lowercase next character.
	\L		  Lowercase till \E.
	\n		  (Logical) newline character.
	\N{}		  Named (Unicode) character.
	\p{}, \pP	  Character with a Unicode property.
	\P{}, \PP	  Character without a Unicode property.
	\Q		  Quotemeta till \E.
	\r		  Return character.
	\s		  Character class for white space.
	\S		  Character class for non white space.
	\t		  Tab character.
	\u		  Titlecase next character.
	\U		  Uppercase till \E.
	\w		  Character class for word characters.
	\W		  Character class for non-word characters.
	\x{}, \x00	  Hexadecimal escape sequence.
	\X		  Extended Unicode "combining character sequence".
	\z		  End of string.
	\Z		  End of string.

       Character Escapes

       Fixed characters

       A handful of characters have a dedicated character escape. The following table shows them,
       along with their code points (in decimal and hex), their ASCII name, the control escape
       (see below) and a short description.

	Seq.  Code Point  ASCII   Cntr	  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 only the backspace character inside a character class. Outside a character
	   class, "\b" is a word/non-word boundary.

       [2] "\n" matches a logical newline. Perl will convert between "\n" and your OSses native
	   newline character when reading from or writing to text files.


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

       Control characters

       "\c" is used to denote a control character; the character following "\c" is the name of
       the control character. For instance, "/\cM/" matches the character control-M (a carriage
       return, code point 13). The case of the character following "\c" doesn't matter: "\cM" and
       "\cm" match the same character.

       Mnemonic: control character.


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

       Named characters

       All Unicode characters have a Unicode name, and characters in various scripts have names
       as well. It is even possible to give your own names to characters.  You can use a charac-
       ter by name by using the "\N{}" construct; the name of the character goes between the
       curly braces. You do have to "use charnames" to load the names of the characters, other-
       wise Perl will complain you use a name it doesn't know about. For more details, see char-

       Mnemonic: Named character.


	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.
	$str =~ /\N{ZHE}\N{KA}/;	     # Match "ZHE" followed by "KA".

       Octal escapes

       Octal escapes consist of a backslash followed by two or three octal digits matching the
       code point of the character you want to use. This allows for 512 characters ("\00" up to
       "\777") that can be expressed this way.	Enough in pre-Unicode days, but most Unicode
       characters cannot be escaped this way.

       Note that a character that is expressed as an octal escape is considered as a character
       without special meaning by the regex engine, and will match "as is".


	$str = "Perl";
	$str =~ /\120/;    # Match, "\120" is "P".
	$str =~ /\120+/;   # Match, "\120" is "P", it is repeated at least once.
	$str =~ /P\053/;   # No match, "\053" is "+" and taken literally.


       Octal escapes potentially clash with backreferences. They both consist of a backslash fol-
       lowed by numbers. So Perl has to use heuristics to determine whether it is a backreference
       or an octal escape. Perl uses the following rules:

       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 (decimal), and Perl already has seen N cap-
	   ture groups, Perl will consider this to be a backreference.	Otherwise, it will con-
	   sider it to be an octal escape. Note that if N > 999, Perl only takes the first three
	   digits for the octal escape; the rest is 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'.

       Hexadecimal escapes

       Hexadecimal escapes start with "\x" and are then either followed by two digit hexadecimal
       number, or a hexadecimal number of arbitrary length surrounded by curly braces. The hexa-
       decimal number is the code point of the character you want to express.

       Note that a character that is expressed as a hexadecimal escape is considered as a charac-
       ter without special meaning by the regex engine, and will match "as is".

       Mnemonic: hexadecimal.


	$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.


       A number of backslash sequences have to do with changing the character, or characters fol-
       lowing them. "\l" will lowercase the character following it, while "\u" will uppercase
       (or, more accurately, titlecase) the character following it. (They perform similar func-
       tionality as 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 pat-
       tern, or the next occurrence of "\E", whatever comes first. They perform similar function-
       ality as the functions "lc" and "uc" do.

       "\Q" is used to escape all characters following, up to the next "\E" or the end of the
       pattern. "\Q" adds a backslash to any character that isn't a letter, digit or underscore.
       This will ensure that any character between "\Q" and "\E" is matched literally, and will
       not be interpreted by the regexp engine.

       Mnemonic: Lowercase, Uppercase, Quotemeta, End.


	$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 word character (letters, digits, underscore).
       "\d" is a character class that matches any digit, while the character class "\s" matches
       any white space character.

       The uppercase variants ("\W", "\D", "\S") are character classes that match any character
       that isn't a word character, digit or white space.

       Mnemonics: word, digit, space

       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 "Back-
       slashed sequences" in perlrecharclass and "Unicode Character Properties" in perlunicode.

       Mnemonic: property.


       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. In Perl 5.8.x and ear-
       lier there is only one way of referring to a backreference, by absolution number. Perl
       5.10 adds the ability to reference relatively and by name.

       Absolute referencing

       A backslash sequence that starts with a backslash and is followed by a number is an abso-
       lute reference (but be aware of the caveat mentioned above).  If the number is N, it
       refers to the Nth set of parenthesis - whatever has been matched by that set of parenthe-
       sis has to be matched by the "\N" as well.


	/(\w+) \1/;    # Finds a duplicated word, (e.g. "cat cat").
	/(.)(.)\2\1/;  # Match a four letter palindrome (e.g. "ABBA").


       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 with "/^/". 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" mod-
	   ifier 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 with "/$/", that is, it matches at the end of the string, or
	   before the newline at the end of the string. If the "/m" modifier is used, then "/$/"
	   matches at internal newlines, but the meaning of "/\Z/" isn't changed by the "/m" mod-
	   ifier. "\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 will not match before a trailing newline. "\z"
	   will only match at the end of the string - regardless of the modifiers used, and not
	   before a newline.

       \G  "\G" is usually only used in combination with the "/g" modifier. If the "/g" modifier
	   is used (and the match is done in scalar context), Perl will remember 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 ended, or the beginning of the 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.

	   Mnemonic: boundary.


	 "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'


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

       \C  "\C" always matches a single octet, even if the source string is encoded in UTF-8 for-
	   mat, and the character to be matched is a multi-octet character.  "\C" was introduced
	   in perl 5.6.

	   Mnemonic: oCtet.

       \X  This matches an extended Unicode combining character sequence, and is equivalent to
	   "(?>\PM\pM*)". "\PM" matches any character that is not considered a Unicode mark char-
	   acter, while "\pM" matches any character that is considered a Unicode mark character;
	   so "\X" matches any non mark character followed by zero or more mark characters. Mark
	   characters include (but are not restricted to) combining characters and vowel signs.

	   "\X" matches quite well what normal (non-Unicode-programmer) usage would consider a
	   single character: for example a base character (the "\PM" above), for example a let-
	   ter, followed by zero or more diacritics, which are combining characters (the "\pM*"

	   Mnemonic: eXtended Unicode character.


	"\x{256}" =~ /^\C\C$/;	  # Match as chr (256) takes 2 octets in UTF-8.

	$str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'.
	$str =~ s/(.)\K\1//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{0307}" =~ /^\X$/	  # \X matches a P with a dot above.

perl v5.8.9				    2007-11-17			       PERLREBACKSLASH(1)

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