PERLREBACKSLASH(1) Perl Programmers Reference Guide PERLREBACKSLASH(1) #
PERLREBACKSLASH(1) Perl Programmers Reference Guide PERLREBACKSLASH(1)
NNAAMMEE #
perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes
DDEESSCCRRIIPPTTIIOONN #
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.
TThhee bbaacckkssllaasshh 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.
AAllll tthhee sseeqquueenncceess aanndd eessccaappeess 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.
\d Match any digit character.
\D Match any character that isn't a digit.
\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 Match any horizontal whitespace character.
\H Match any character that isn't 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 Match any character but newline. Not in [].
\N{} Named or numbered (Unicode) character or sequence.
\o{} Octal escape sequence.
\p{}, \pP Match any character with the given Unicode property.
\P{}, \PP Match any character without the given property.
\Q Quote (disable) pattern metacharacters till \E. Not
in [].
\r Return character.
\R Generic new line. Not in [].
\s Match any whitespace character.
\S Match any character that isn't a whitespace.
\t Tab character.
\u Titlecase next character. Not in [].
\U Uppercase till \E. Not in [].
\v Match any vertical whitespace character.
\V Match any character that isn't vertical whitespace
\w Match any word character.
\W Match any character that isn't a word character.
\x{}, \x00 Hexadecimal escape sequence.
\X Unicode "extended grapheme cluster". Not in [].
\z End of string. Not in [].
\Z End of string. Not in [].
CChhaarraacctteerr EEssccaappeess _F_i_x_e_d _c_h_a_r_a_c_t_e_r_s
A handful of characters have a dedicated _c_h_a_r_a_c_t_e_r _e_s_c_a_p_e. 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.
_C_o_n_t_r_o_l _c_h_a_r_a_c_t_e_r_s
"\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{_N_A_M_E}" 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).
_N_a_m_e_d _o_r _n_u_m_b_e_r_e_d _c_h_a_r_a_c_t_e_r_s _a_n_d _c_h_a_r_a_c_t_e_r _s_e_q_u_e_n_c_e_s
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+_c_o_d_e
_p_o_i_n_t}", where _c_o_d_e _p_o_i_n_t 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).
Blanks may freely be inserted adjacent to but within the braces enclosing
the name or code point. So "\N{ U+0041 }" is perfectly legal.
It is even possible to give your own names to characters and character
sequences by using the charnames module. These custom names are
lexically scoped, and so a given code point may have different names in
different scopes. The name used is what is in effect at the time the
"\N{}" is expanded. For patterns in double-quotish context, that means
at the time the pattern is parsed. But for patterns that are delimitted
by single quotes, the expansion is deferred until pattern compilation
time, which may very well have a different "charnames" translator in
effect.
(There is an expanded internal form that you may see in debug output:
"\N{U+_c_o_d_e _p_o_i_n_t._c_o_d_e _p_o_i_n_t...}". The "..." means any number of these
_c_o_d_e _p_o_i_n_ts 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".
_O_c_t_a_l _e_s_c_a_p_e_s
There are two forms of octal escapes. Each is used to specify a
character by its code point specified in base 8.
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).
/\o{ 400}/ # LATIN CAPITAL LETTER A WITH MACRON
/\o{ 400 }/ # Same. These show blanks are allowed adjacent to
# the braces
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".
_H_e_x_a_d_e_c_i_m_a_l _e_s_c_a_p_e_s
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: he_xadecimal.
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.
/\x{ 263b }/ # Same, showing optional blanks adjacent to the
# braces
MMooddiiffiieerrss 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 /\(Miranda\)/
CChhaarraacctteerr ccllaasssseess 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 _w_o_r_d 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.
_U_n_i_c_o_d_e _c_l_a_s_s_e_s
"\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.
RReeffeerreenncciinngg 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 _b_a_c_k_r_e_f_e_r_e_n_c_e: absolutely, relatively, and by name.
_A_b_s_o_l_u_t_e _r_e_f_e_r_e_n_c_i_n_g
Either "\g_N" (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 "\g_N" refers to whatever has
been matched by that set of parentheses. Thus "\g1" refers to the first
capture group in the regex.
The "\g_N" 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.
/(\w+) \g{1}/; # Same, using the safer braced notation
/(\w+) \g{ 1 }/;# Same, showing optional blanks adjacent to the braces
/(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
_R_e_l_a_t_i_v_e _r_e_f_e_r_e_n_c_i_n_g
"\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)
\g{ -3 } # Same, showing optional blanks adjacent to the braces
)
/x; # Matches "ABBA".
my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
/$qr$qr/ # Matches 'ababcdcd'.
_N_a_m_e_d _r_e_f_e_r_e_n_c_i_n_g
"\g{_n_a_m_e}" (starting in Perl 5.10.0) can be used to back refer to a named
capture group, dispensing completely with having to think about capture
buffer positions.
To be compatible with .Net regular expressions, "\g{name}" may also be
written as "\k{name}", "\k<name>" or "\k'name'".
To prevent any ambiguity, _n_a_m_e must not start with a digit nor contain a
hyphen.
Examples
/(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
/(?<word>\w+) \k{word}/ # Same.
/(?<word>\w+) \g{ word }/ # Same, showing optional blanks adjacent to
# the braces
/(?<word>\w+) \k{ word }/ # Same.
/(?<word>\w+) \k<word>/ # Same. There are no braces, so no blanks
# are permitted
/(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
# Match a four letter palindrome (e.g.
# “ABBA”) #
AAsssseerrttiioonnss Assertions are conditions that have to be true; they don’t actually match parts of the substring. There are six assertions that are written as backslash sequences.
\A "\A" only matches at the beginning of the string. If the "/m"
modifier isn't used, then "/\A/" is equivalent to "/^/". However, if
the "/m" modifier is used, then "/^/" matches internal newlines, but
the meaning of "/\A/" isn't changed by the "/m" modifier. "\A"
matches at the beginning of the string regardless whether the "/m"
modifier is used.
\z, \Z
"\z" and "\Z" match at the end of the string. If the "/m" modifier
isn't used, then "/\Z/" is equivalent to "/$/"; 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 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 "{" is available in all
Perls. It 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
<https://www.unicode.org/reports/tr14/> and
<https://www.unicode.org/reports/tr29/>. The boundary types 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{lb}"
This matches according to the default Unicode Line Breaking
Algorithm (<https://www.unicode.org/reports/tr14/>), as
customized in that document (Example 7 of revision 35
<https://www.unicode.org/reports/tr14/tr14-35.html#Example7>) for
better handling of numeric expressions.
This is suitable for many purposes, but the Unicode::LineBreak
module is available on CPAN that provides many more features,
including customization.
"\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
<https://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 sentences. "\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", but tailored to Perl
expectations. 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
<https://www.unicode.org/reports/tr29/>.
The current Unicode definition of a Word Boundary matches between
every white space character. Perl tailors this, starting in
version 5.24, to generally not break up spans of white space,
just as plain "\b" has always functioned. This allows "\b{wb}"
to be a drop-in replacement for "\b", but with generally better
results for natural language processing. (The exception to this
tailoring is when a span of white space is immediately followed
by something like U+0303, COMBINING TILDE. If the final space
character in the span is a horizontal white space, it is broken
out so that it attaches instead to the combining character. To
be precise, if a span of white space that ends in a horizontal
space has the character immediately following it have any of the
Word Boundary property values "Extend", "Format" or "ZWJ", the
boundary between the final horizontal space character and the
rest of the span matches "\b{wb}". In all other cases the
boundary between two white space characters matches "\B{wb}".)
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. For example, some languages,
such as Japanese and Thai, require dictionary lookup to accurately
determine word boundaries.
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|)|.|"
MMiisscc Here we document the backslash sequences that don’t fall in one of the categories above. These are:
\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 nnoott 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 _g_e_n_e_r_i_c _n_e_w_l_i_n_e; 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 _e_x_t_e_n_d_e_d _g_r_a_p_h_e_m_e _c_l_u_s_t_e_r.
"\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: e_Xtended 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.
perl v5.36.3 2023-02-15 PERLREBACKSLASH(1)