perlrebackslash.1

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.PP
Octal escapes potentially clash with backreferences. 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:
.IP "1." 4
If the backslash is followed by a single digit, it's a backreference.
.IP "2." 4
If the first digit following the backslash is a 0, it's an octal escape.
.IP "3." 4
If the number following the backslash is N (decimal), and Perl already has
seen N capture groups, Perl will consider this to be a backreference.
Otherwise, it will consider 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.
.Sp
.Vb 6
\& my $pat  = "(" x 999;
\&    $pat .= "a";
\&    $pat .= ")" x 999;
\& /^($pat)\e1000$/;   #  Matches \*(Aqaa\*(Aq; there are 1000 capture groups.
\& /^$pat\e1000$/;     #  Matches \*(Aqa@0\*(Aq; there are 999 capture groups
\&                    #    and \e1000 is seen as \e100 (a \*(Aq@\*(Aq) and a \*(Aq0\*(Aq.
.Ve
.PP
\fIHexadecimal escapes\fR
.IX Subsection "Hexadecimal escapes"
.PP
Hexadecimal escapes start with \f(CW\*(C`\ex\*(C'\fR and are then either followed by
two digit hexadecimal 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.
.PP
Note that a character that is expressed as a hexadecimal escape is considered
as a character without special meaning by the regex engine, and will match
\&\*(L"as is\*(R".
.PP
Mnemonic: he\fIx\fRadecimal.
.PP
Examples
.IX Subsection "Examples"
.PP
.Vb 4
\& $str = "Perl";
\& $str =~ /\ex50/;    # Match, "\ex50" is "P".
\& $str =~ /\ex50+/;   # Match, "\ex50" is "P", it is repeated at least once.
\& $str =~ /P\ex2B/;   # No match, "\ex2B" is "+" and taken literally.
\&
\& /\ex{2603}\ex{2602}/ # Snowman with an umbrella.
\&                    # The Unicode character 2603 is a snowman,
\&                    # the Unicode character 2602 is an umbrella.
\& /\ex{263B}/         # Black smiling face.
\& /\ex{263b}/         # Same, the hex digits A \- F are case insensitive.
.Ve
.Sh "Modifiers"
.IX Subsection "Modifiers"
A number of backslash sequences have to do with changing the character,
or characters following them. \f(CW\*(C`\el\*(C'\fR will lowercase the character following
it, while \f(CW\*(C`\eu\*(C'\fR will uppercase (or, more accurately, titlecase) the
character following it. (They perform similar functionality as the
functions \f(CW\*(C`lcfirst\*(C'\fR and \f(CW\*(C`ucfirst\*(C'\fR).
.PP
To uppercase or lowercase several characters, one might want to use
\&\f(CW\*(C`\eL\*(C'\fR or \f(CW\*(C`\eU\*(C'\fR, which will lowercase/uppercase all characters following
them, until either the end of the pattern, or the next occurrence of
\&\f(CW\*(C`\eE\*(C'\fR, whatever comes first. They perform similar functionality as the
functions \f(CW\*(C`lc\*(C'\fR and \f(CW\*(C`uc\*(C'\fR do.
.PP
\&\f(CW\*(C`\eQ\*(C'\fR is used to escape all characters following, up to the next \f(CW\*(C`\eE\*(C'\fR
or the end of the pattern. \f(CW\*(C`\eQ\*(C'\fR adds a backslash to any character that
isn't a letter, digit or underscore. This will ensure that any character
between \f(CW\*(C`\eQ\*(C'\fR and \f(CW\*(C`\eE\*(C'\fR is matched literally, and will not be interpreted
by the regexp engine.
.PP
Mnemonic: \fIL\fRowercase, \fIU\fRppercase, \fIQ\fRuotemeta, \fIE\fRnd.
.PP
Examples
.IX Subsection "Examples"
.PP
.Vb 7
\& $sid     = "sid";
\& $greg    = "GrEg";
\& $miranda = "(Miranda)";
\& $str     =~ /\eu$sid/;        # Matches \*(AqSid\*(Aq
\& $str     =~ /\eL$greg/;       # Matches \*(Aqgreg\*(Aq
\& $str     =~ /\eQ$miranda\eE/;  # Matches \*(Aq(Miranda)\*(Aq, as if the pattern
\&                              #   had been written as /\e(Miranda\e)/
.Ve
.Sh "Character classes"
.IX Subsection "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.
.PP
\&\f(CW\*(C`\ew\*(C'\fR is a character class that matches any \fIword\fR character (letters,
digits, underscore). \f(CW\*(C`\ed\*(C'\fR is a character class that matches any digit,
while the character class \f(CW\*(C`\es\*(C'\fR matches any white space character.
New in perl 5.10.0 are the classes \f(CW\*(C`\eh\*(C'\fR and \f(CW\*(C`\ev\*(C'\fR which match horizontal
and vertical white space characters.
.PP
The uppercase variants (\f(CW\*(C`\eW\*(C'\fR, \f(CW\*(C`\eD\*(C'\fR, \f(CW\*(C`\eS\*(C'\fR, \f(CW\*(C`\eH\*(C'\fR, and \f(CW\*(C`\eV\*(C'\fR) are
character classes that match any character that isn't a word character,
digit, white space, horizontal white space or vertical white space.
.PP
Mnemonics: \fIw\fRord, \fId\fRigit, \fIs\fRpace, \fIh\fRorizontal, \fIv\fRertical.
.PP
\fIUnicode classes\fR
.IX Subsection "Unicode classes"
.PP
\&\f(CW\*(C`\epP\*(C'\fR (where \f(CW\*(C`P\*(C'\fR is a single letter) and \f(CW\*(C`\ep{Property}\*(C'\fR are used to
match a character that matches the given Unicode property; properties
include things like \*(L"letter\*(R", or \*(L"thai character\*(R". Capitalizing the
sequence to \f(CW\*(C`\ePP\*(C'\fR and \f(CW\*(C`\eP{Property}\*(C'\fR make the sequence match a character
that doesn't match the given Unicode property. For more details, see
\&\*(L"Backslashed sequences\*(R" in perlrecharclass and
\&\*(L"Unicode Character Properties\*(R" in perlunicode.
.PP
Mnemonic: \fIp\fRroperty.
.Sh "Referencing"
.IX Subsection "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 \fIbackreference\fR:
absolutely, relatively, and by name.
.PP
\fIAbsolute referencing\fR
.IX Subsection "Absolute referencing"
.PP
A backslash sequence that starts with a backslash and is followed by a
number is an absolute reference (but be aware of the caveat mentioned above).
If the number is \fIN\fR, it refers to the Nth set of parenthesis \- whatever
has been matched by that set of parenthesis has to be matched by the \f(CW\*(C`\eN\*(C'\fR
as well.
.PP
Examples
.IX Subsection "Examples"
.PP
.Vb 2
\& /(\ew+) \e1/;    # Finds a duplicated word, (e.g. "cat cat").
\& /(.)(.)\e2\e1/;  # Match a four letter palindrome (e.g. "ABBA").
.Ve
.PP
\fIRelative referencing\fR
.IX Subsection "Relative referencing"
.PP
New in perl 5.10.0 is a different way of referring to capture buffers: \f(CW\*(C`\eg\*(C'\fR.
\&\f(CW\*(C`\eg\*(C'\fR takes a number as argument, with the number in curly braces (the
braces are optional). If the number (N) does not have a sign, it's a reference
to the Nth capture group (so \f(CW\*(C`\eg{2}\*(C'\fR is equivalent to \f(CW\*(C`\e2\*(C'\fR \- except that
\&\f(CW\*(C`\eg\*(C'\fR always refers to a capture group and will never be seen as an octal
escape). If the number is negative, the reference is relative, referring to
the Nth group before the \f(CW\*(C`\eg{\-N}\*(C'\fR.
.PP
The big advantage of \f(CW\*(C`\eg{\-N}\*(C'\fR 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.
.PP
Mnemonic: \fIg\fRroup.
.PP
Examples
.IX Subsection "Examples"
.PP
.Vb 7
\& /(A)        # Buffer 1
\&  (          # Buffer 2
\&    (B)      # Buffer 3
\&    \eg{\-1}   # Refers to buffer 3 (B)
\&    \eg{\-3}   # Refers to buffer 1 (A)
\&  )
\& /x;         # Matches "ABBA".
\&
\& my $qr = qr /(.)(.)\eg{\-2}\eg{\-1}/;  # Matches \*(Aqabab\*(Aq, \*(Aqcdcd\*(Aq, etc.
\& /$qr$qr/                           # Matches \*(Aqababcdcd\*(Aq.
.Ve
.PP
\fINamed referencing\fR
.IX Subsection "Named referencing"
.PP
Also new in perl 5.10.0 is the use of named capture buffers, which can be
referred to by name. This is done with \f(CW\*(C`\eg{name}\*(C'\fR, which is a
backreference to the capture buffer with the name \fIname\fR.
.PP
To be compatible with .Net regular expressions, \f(CW\*(C`\eg{name}\*(C'\fR may also be
written as \f(CW\*(C`\ek{name}\*(C'\fR, \f(CW\*(C`\ek<name>\*(C'\fR or \f(CW\*(C`\ek\*(Aqname\*(Aq\*(C'\fR.
.PP
Note that \f(CW\*(C`\eg{}\*(C'\fR has the potential to be ambiguous, as it could be a named
reference, or an absolute or relative reference (if its argument is numeric).
However, names are not allowed to start with digits, nor are allowed to
contain a hyphen, so there is no ambiguity.
.PP
Examples
.IX Subsection "Examples"
.PP
.Vb 5
\& /(?<word>\ew+) \eg{word}/ # Finds duplicated word, (e.g. "cat cat")
\& /(?<word>\ew+) \ek{word}/ # Same.
\& /(?<word>\ew+) \ek<word>/ # Same.
\& /(?<letter1>.)(?<letter2>.)\eg{letter2}\eg{letter1}/
\&                         # Match a four letter palindrome (e.g. "ABBA")
.Ve
.Sh "Assertions"
.IX Subsection "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.
.IP "\eA" 4
.IX Item "A"
\&\f(CW\*(C`\eA\*(C'\fR only matches at the beginning of the string. If the \f(CW\*(C`/m\*(C'\fR modifier
isn't used, then \f(CW\*(C`/\eA/\*(C'\fR is equivalent with \f(CW\*(C`/^/\*(C'\fR. However, if the \f(CW\*(C`/m\*(C'\fR
modifier is used, then \f(CW\*(C`/^/\*(C'\fR matches internal newlines, but the meaning
of \f(CW\*(C`/\eA/\*(C'\fR isn't changed by the \f(CW\*(C`/m\*(C'\fR modifier. \f(CW\*(C`\eA\*(C'\fR matches at the beginning
of the string regardless whether the \f(CW\*(C`/m\*(C'\fR modifier is used.
.IP "\ez, \eZ" 4
.IX Item "z, Z"
\&\f(CW\*(C`\ez\*(C'\fR and \f(CW\*(C`\eZ\*(C'\fR match at the end of the string. If the \f(CW\*(C`/m\*(C'\fR modifier isn't
used, then \f(CW\*(C`/\eZ/\*(C'\fR is equivalent with \f(CW\*(C`/$/\*(C'\fR, that is, it matches at the
end of the string, or before the newline at the end of the string. If the
\&\f(CW\*(C`/m\*(C'\fR modifier is used, then \f(CW\*(C`/$/\*(C'\fR matches at internal newlines, but the
meaning of \f(CW\*(C`/\eZ/\*(C'\fR isn't changed by the \f(CW\*(C`/m\*(C'\fR modifier. \f(CW\*(C`\eZ\*(C'\fR matches at
the end of the string (or just before a trailing newline) regardless whether
the \f(CW\*(C`/m\*(C'\fR modifier is used.
.Sp
\&\f(CW\*(C`\ez\*(C'\fR is just like \f(CW\*(C`\eZ\*(C'\fR, except that it will not match before a trailing
newline. \f(CW\*(C`\ez\*(C'\fR will only match at the end of the string \- regardless of the
modifiers used, and not before a newline.
.IP "\eG" 4
.IX Item "G"
\&\f(CW\*(C`\eG\*(C'\fR is usually only used in combination with the \f(CW\*(C`/g\*(C'\fR modifier. If the
\&\f(CW\*(C`/g\*(C'\fR 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.
.Sp
\&\f(CW\*(C`\eG\*(C'\fR matches the point where the previous match ended, or the beginning
of the string if there was no previous match.
.Sp
Mnemonic: \fIG\fRlobal.
.IP "\eb, \eB" 4
.IX Item "b, B"
\&\f(CW\*(C`\eb\*(C'\fR matches at any place between a word and a non-word character; \f(CW\*(C`\eB\*(C'\fR
matches at any place between characters where \f(CW\*(C`\eb\*(C'\fR doesn't match. \f(CW\*(C`\eb\*(C'\fR
and \f(CW\*(C`\eB\*(C'\fR assume there's a non-word character before the beginning and after
the end of the source string; so \f(CW\*(C`\eb\*(C'\fR will match at the beginning (or end)
of the source string if the source string begins (or ends) with a word
character. Otherwise, \f(CW\*(C`\eB\*(C'\fR will match.
.Sp
Mnemonic: \fIb\fRoundary.
.PP
Examples
.IX Subsection "Examples"
.PP
.Vb 4
\&  "cat"   =~ /\eAcat/;     # Match.
\&  "cat"   =~ /cat\eZ/;     # Match.
\&  "cat\en" =~ /cat\eZ/;     # Match.
\&  "cat\en" =~ /cat\ez/;     # No match.
\&
\&  "cat"   =~ /\ebcat\eb/;   # Matches.
\&  "cats"  =~ /\ebcat\eb/;   # No match.
\&  "cat"   =~ /\ebcat\eB/;   # No match.
\&  "cats"  =~ /\ebcat\eB/;   # Match.
\&
\&  while ("cat dog" =~ /(\ew+)/g) {
\&      print $1;           # Prints \*(Aqcatdog\*(Aq
\&  }
\&  while ("cat dog" =~ /\eG(\ew+)/g) {
\&      print $1;           # Prints \*(Aqcat\*(Aq
\&  }
.Ve
.Sh "Misc"
.IX Subsection "Misc"
Here we document the backslash sequences that don't fall in one of the
categories above. They are:
.IP "\eC" 4
.IX Item "C"
\&\f(CW\*(C`\eC\*(C'\fR always matches a single octet, even if the source string is encoded
in \s-1UTF\-8\s0 format, and the character to be matched is a multi-octet character.
\&\f(CW\*(C`\eC\*(C'\fR was introduced in perl 5.6.
.Sp
Mnemonic: o\fIC\fRtet.
.IP "\eK" 4
.IX Item "K"
This is new in perl 5.10.0. Anything that is matched left of \f(CW\*(C`\eK\*(C'\fR is
not included in \f(CW$&\fR \- and will not be replaced if the pattern is
used in a substitution. This will allow you to write \f(CW\*(C`s/PAT1 \eK PAT2/REPL/x\*(C'\fR
instead of \f(CW\*(C`s/(PAT1) PAT2/${1}REPL/x\*(C'\fR or \f(CW\*(C`s/(?<=PAT1) PAT2/REPL/x\*(C'\fR.
.Sp
Mnemonic: \fIK\fReep.
.IP "\eR" 4
.IX Item "R"
\&\f(CW\*(C`\eR\*(C'\fR matches a \fIgeneric newline\fR, that is, anything that is considered
a newline by Unicode. This includes all characters matched by \f(CW\*(C`\ev\*(C'\fR
(vertical white space), and the multi character sequence \f(CW"\ex0D\ex0A"\fR
(carriage return followed by a line feed, aka the network newline, or
the newline used in Windows text files). \f(CW\*(C`\eR\*(C'\fR is equivalent with
\&\f(CW\*(C`(?>\ex0D\ex0A)|\ev)\*(C'\fR. Since \f(CW\*(C`\eR\*(C'\fR can match a more than one character,
it cannot be put inside a bracketed character class; \f(CW\*(C`/[\eR]/\*(C'\fR is an error.
\&\f(CW\*(C`\eR\*(C'\fR was introduced in perl 5.10.0.
.Sp
Mnemonic: none really. \f(CW\*(C`\eR\*(C'\fR was picked because \s-1PCRE\s0 already uses \f(CW\*(C`\eR\*(C'\fR,
and more importantly because Unicode recommends such a regular expression
metacharacter, and suggests \f(CW\*(C`\eR\*(C'\fR as the notation.
.IP "\eX" 4
.IX Item "X"
This matches an extended Unicode \fIcombining character sequence\fR, and
is equivalent to \f(CW\*(C`(?>\ePM\epM*)\*(C'\fR. \f(CW\*(C`\ePM\*(C'\fR matches any character that is
not considered a Unicode mark character, while \f(CW\*(C`\epM\*(C'\fR matches any character
that is considered a Unicode mark character; so \f(CW\*(C`\eX\*(C'\fR matches any non
mark character followed by zero or more mark characters. Mark characters
include (but are not restricted to) \fIcombining characters\fR and
\&\fIvowel signs\fR.
.Sp
\&\f(CW\*(C`\eX\*(C'\fR matches quite well what normal (non-Unicode-programmer) usage
would consider a single character: for example a base character
(the \f(CW\*(C`\ePM\*(C'\fR above), for example a letter, followed by zero or more
diacritics, which are \fIcombining characters\fR (the \f(CW\*(C`\epM*\*(C'\fR above).
.Sp
Mnemonic: e\fIX\fRtended Unicode character.
.PP
Examples
.IX Subsection "Examples"
.PP
.Vb 1
\& "\ex{256}" =~ /^\eC\eC$/;    # Match as chr (256) takes 2 octets in UTF\-8.
\&
\& $str =~ s/foo\eKbar/baz/g; # Change any \*(Aqbar\*(Aq following a \*(Aqfoo\*(Aq to \*(Aqbaz\*(Aq.
\& $str =~ s/(.)\eK\e1//g;     # Delete duplicated characters.
\&
\& "\en"   =~ /^\eR$/;         # Match, \en   is a generic newline.
\& "\er"   =~ /^\eR$/;         # Match, \er   is a generic newline.
\& "\er\en" =~ /^\eR$/;         # Match, \er\en is a generic newline.
\&
\& "P\ex{0307}" =~ /^\eX$/     # \eX matches a P with a dot above.
.Ve