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PERLREQUICK(1)	       Perl Programmers	Reference Guide		PERLREQUICK(1)

NAME
       perlrequick - Perl regular expressions quick start

DESCRIPTION
       This page covers	the very basics	of understanding, creating and using
       regular expressions ('regexes') in Perl.

The Guide
       This page assumes you already know things, like what a "pattern"	is,
       and the basic syntax of using them.  If you don't, see perlretut.

   Simple word matching
       The simplest regex is simply a word, or more generally, a string	of
       characters.  A regex consisting of a word matches any string that
       contains	that word:

	   "Hello World" =~ /World/;  #	matches

       In this statement, "World" is a regex and the "//" enclosing "/World/"
       tells Perl to search a string for a match.  The operator	"=~"
       associates the string with the regex match and produces a true value if
       the regex matched, or false if the regex	did not	match.	In our case,
       "World" matches the second word in "Hello World", so the	expression is
       true.  This idea	has several variations.

       Expressions like	this are useful	in conditionals:

	   print "It matches\n"	if "Hello World" =~ /World/;

       The sense of the	match can be reversed by using "!~" operator:

	   print "It doesn't match\n" if "Hello	World" !~ /World/;

       The literal string in the regex can be replaced by a variable:

	   $greeting = "World";
	   print "It matches\n"	if "Hello World" =~ /$greeting/;

       If you're matching against $_, the "$_ =~" part can be omitted:

	   $_ =	"Hello World";
	   print "It matches\n"	if /World/;

       Finally,	the "//" default delimiters for	a match	can be changed to
       arbitrary delimiters by putting an 'm' out front:

	   "Hello World" =~ m!World!;	# matches, delimited by	'!'
	   "Hello World" =~ m{World};	# matches, note	the matching '{}'
	   "/usr/bin/perl" =~ m"/perl";	# matches after	'/usr/bin',
					# '/' becomes an ordinary char

       Regexes must match a part of the	string exactly in order	for the
       statement to be true:

	   "Hello World" =~ /world/;  #	doesn't	match, case sensitive
	   "Hello World" =~ /o W/;    #	matches, ' ' is	an ordinary char
	   "Hello World" =~ /World /; #	doesn't	match, no ' ' at end

       Perl will always	match at the earliest possible point in	the string:

	   "Hello World" =~ /o/;       # matches 'o' in	'Hello'
	   "That hat is	red" =~	/hat/; # matches 'hat' in 'That'

       Not all characters can be used 'as is' in a match.  Some	characters,
       called metacharacters, are considered special, and reserved for use in
       regex notation.	The metacharacters are

	   {}[]()^$.|*+?\

       A metacharacter can be matched literally	by putting a backslash before
       it:

	   "2+2=4" =~ /2+2/;	# doesn't match, + is a	metacharacter
	   "2+2=4" =~ /2\+2/;	# matches, \+ is treated like an ordinary +
	   'C:\WIN32' =~ /C:\\WIN/;			  # matches
	   "/usr/bin/perl" =~ /\/usr\/bin\/perl/;  # matches

       In the last regex, the forward slash '/'	is also	backslashed, because
       it is used to delimit the regex.

       Most of the metacharacters aren't always	special, and other characters
       (such as	the ones delimiting the	pattern) become	special	under various
       circumstances.  This can	be confusing and lead to unexpected results.
       "use re 'strict'" can notify you	of potential pitfalls.

       Non-printable ASCII characters are represented by escape	sequences.
       Common examples are "\t"	for a tab, "\n"	for a newline, and "\r"	for a
       carriage	return.	 Arbitrary bytes are represented by octal escape
       sequences, e.g.,	"\033",	or hexadecimal escape sequences, e.g., "\x1B":

	   "1000\t2000"	=~ m(0\t2)  # matches
	   "cat" =~ /\143\x61\x74/  # matches in ASCII,	but
				    # a	weird way to spell cat

       Regexes are treated mostly as double-quoted strings, so variable
       substitution works:

	   $foo	= 'house';
	   'cathouse' =~ /cat$foo/;   #	matches
	   'housecat' =~ /${foo}cat/; #	matches

       With all	of the regexes above, if the regex matched anywhere in the
       string, it was considered a match.  To specify where it should match,
       we would	use the	anchor metacharacters "^" and "$".  The	anchor "^"
       means match at the beginning of the string and the anchor "$" means
       match at	the end	of the string, or before a newline at the end of the
       string.	Some examples:

	   "housekeeper" =~ /keeper/;	      #	matches
	   "housekeeper" =~ /^keeper/;	      #	doesn't	match
	   "housekeeper" =~ /keeper$/;	      #	matches
	   "housekeeper\n" =~ /keeper$/;      #	matches
	   "housekeeper" =~ /^housekeeper$/;  #	matches

   Using character classes
       A character class allows	a set of possible characters, rather than just
       a single	character, to match at a particular point in a regex.  There
       are a number of different types of character classes, but usually when
       people use this term, they are referring	to the type described in this
       section,	which are technically called "Bracketed	character classes",
       because they are	denoted	by brackets "[...]", with the set of
       characters to be	possibly matched inside.  But we'll drop the
       "bracketed" below to correspond with common usage.  Here	are some
       examples	of (bracketed) character classes:

	   /cat/;	     # matches 'cat'
	   /[bcr]at/;	     # matches 'bat', 'cat', or	'rat'
	   "abc" =~ /[cab]/; # matches 'a'

       In the last statement, even though 'c' is the first character in	the
       class, the earliest point at which the regex can	match is 'a'.

	   /[yY][eE][sS]/; # match 'yes' in a case-insensitive way
			   # 'yes', 'Yes', 'YES', etc.
	   /yes/i;	   # also match	'yes' in a case-insensitive way

       The last	example	shows a	match with an 'i' modifier, which makes	the
       match case-insensitive.

       Character classes also have ordinary and	special	characters, but	the
       sets of ordinary	and special characters inside a	character class	are
       different than those outside a character	class.	The special characters
       for a character class are "-]\^$" and are matched using an escape:

	  /[\]c]def/; #	matches	']def' or 'cdef'
	  $x = 'bcr';
	  /[$x]at/;   #	matches	'bat, 'cat', or	'rat'
	  /[\$x]at/;  #	matches	'$at' or 'xat'
	  /[\\$x]at/; #	matches	'\at', 'bat, 'cat', or 'rat'

       The special character '-' acts as a range operator within character
       classes,	so that	the unwieldy "[0123456789]" and	"[abc...xyz]" become
       the svelte "[0-9]" and "[a-z]":

	   /item[0-9]/;	 # matches 'item0' or ... or 'item9'
	   /[0-9a-fA-F]/;  # matches a hexadecimal digit

       If '-' is the first or last character in	a character class, it is
       treated as an ordinary character.

       The special character "^" in the	first position of a character class
       denotes a negated character class, which	matches	any character but
       those in	the brackets.  Both "[...]" and	"[^...]" must match a
       character, or the match fails.  Then

	   /[^a]at/;  #	doesn't	match 'aat' or 'at', but matches
		      #	all other 'bat', 'cat, '0at', '%at', etc.
	   /[^0-9]/;  #	matches	a non-numeric character
	   /[a^]at/;  #	matches	'aat' or '^at';	here '^' is ordinary

       Perl has	several	abbreviations for common character classes. (These
       definitions are those that Perl uses in ASCII-safe mode with the	"/a"
       modifier.  Otherwise they could match many more non-ASCII Unicode
       characters as well.  See	"Backslash sequences" in perlrecharclass for
       details.)

       o   \d is a digit and represents

	       [0-9]

       o   \s is a whitespace character	and represents

	       [\ \t\r\n\f]

       o   \w is a word	character (alphanumeric	or _) and represents

	       [0-9a-zA-Z_]

       o   \D is a negated \d; it represents any character but a digit

	       [^0-9]

       o   \S is a negated \s; it represents any non-whitespace	character

	       [^\s]

       o   \W is a negated \w; it represents any non-word character

	       [^\w]

       o   The period '.' matches any character	but "\n"

       The "\d\s\w\D\S\W" abbreviations	can be used both inside	and outside of
       character classes.  Here	are some in use:

	   /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
	   /[\d\s]/;	     # matches any digit or whitespace character
	   /\w\W\w/;	     # matches a word char, followed by	a
			     # non-word	char, followed by a word char
	   /..rt/;	     # matches any two chars, followed by 'rt'
	   /end\./;	     # matches 'end.'
	   /end[.]/;	     # same thing, matches 'end.'

       The word	anchor	"\b" matches a boundary	between	a word character and a
       non-word	character "\w\W" or "\W\w":

	   $x =	"Housecat catenates house and cat";
	   $x =~ /\bcat/;  # matches cat in 'catenates'
	   $x =~ /cat\b/;  # matches cat in 'housecat'
	   $x =~ /\bcat\b/;  # matches 'cat' at	end of string

       In the last example, the	end of the string is considered	a word
       boundary.

       For natural language processing (so that, for example, apostrophes are
       included	in words), use instead "\b{wb}"

	   "don't" =~ /	.+? \b{wb} /x;	# matches the whole string

   Matching this or that
       We can match different character	strings	with the alternation
       metacharacter '|'.  To match "dog" or "cat", we form the	regex
       "dog|cat".  As before, Perl will	try to match the regex at the earliest
       possible	point in the string.  At each character	position, Perl will
       first try to match the first alternative, "dog".	 If "dog" doesn't
       match, Perl will	then try the next alternative, "cat".  If "cat"
       doesn't match either, then the match fails and Perl moves to the	next
       position	in the string.	Some examples:

	   "cats and dogs" =~ /cat|dog|bird/;  # matches "cat"
	   "cats and dogs" =~ /dog|cat|bird/;  # matches "cat"

       Even though "dog" is the	first alternative in the second	regex, "cat"
       is able to match	earlier	in the string.

	   "cats"	   =~ /c|ca|cat|cats/; # matches "c"
	   "cats"	   =~ /cats|cat|ca|c/; # matches "cats"

       At a given character position, the first	alternative that allows	the
       regex match to succeed will be the one that matches. Here, all the
       alternatives match at the first string position,	so the first matches.

   Grouping things and hierarchical matching
       The grouping metacharacters "()"	allow a	part of	a regex	to be treated
       as a single unit.  Parts	of a regex are grouped by enclosing them in
       parentheses.  The regex "house(cat|keeper)" means match "house"
       followed	by either "cat"	or "keeper".  Some more	examples are

	   /(a|b)b/;	# matches 'ab' or 'bb'
	   /(^a|b)c/;	# matches 'ac' at start	of string or 'bc' anywhere

	   /house(cat|)/;  # matches either 'housecat' or 'house'
	   /house(cat(s|)|)/;  # matches either	'housecats' or 'housecat' or
			       # 'house'.  Note	groups can be nested.

	   "20"	=~ /(19|20|)\d\d/;  # matches the null alternative '()\d\d',
				    # because '20\d\d' can't match

   Extracting matches
       The grouping metacharacters "()"	also allow the extraction of the parts
       of a string that	matched.  For each grouping, the part that matched
       inside goes into	the special variables $1, $2, etc.  They can be	used
       just as ordinary	variables:

	   # extract hours, minutes, seconds
	   $time =~ /(\d\d):(\d\d):(\d\d)/;  # match hh:mm:ss format
	   $hours = $1;
	   $minutes = $2;
	   $seconds = $3;

       In list context,	a match	"/regex/" with groupings will return the list
       of matched values "($1,$2,...)".	 So we could rewrite it	as

	   ($hours, $minutes, $second) = ($time	=~ /(\d\d):(\d\d):(\d\d)/);

       If the groupings	in a regex are nested, $1 gets the group with the
       leftmost	opening	parenthesis, $2	the next opening parenthesis, etc.
       For example, here is a complex regex and	the matching variables
       indicated below it:

	   /(ab(cd|ef)((gi)|j))/;
	    1  2      34

       Associated with the matching variables $1, $2, ... are the
       backreferences "\g1", "\g2", ...	 Backreferences	are matching variables
       that can	be used	inside a regex:

	   /(\w\w\w)\s\g1/; # find sequences like 'the the' in string

       $1, $2, ... should only be used outside of a regex, and "\g1", "\g2",
       ... only	inside a regex.

   Matching repetitions
       The quantifier metacharacters "?", "*", "+", and	"{}" allow us to
       determine the number of repeats of a portion of a regex we consider to
       be a match.  Quantifiers	are put	immediately after the character,
       character class,	or grouping that we want to specify.  They have	the
       following meanings:

       o   "a?"	= match	'a' 1 or 0 times

       o   "a*"	= match	'a' 0 or more times, i.e., any number of times

       o   "a+"	= match	'a' 1 or more times, i.e., at least once

       o   "a{n,m}" = match at least "n" times,	but not	more than "m" times.

       o   "a{n,}" = match at least "n"	or more	times

       o   "a{,n}" = match "n" times or	fewer

       o   "a{n}" = match exactly "n" times

       Here are	some examples:

	   /[a-z]+\s+\d*/;  # match a lowercase	word, at least some space, and
			    # any number of digits
	   /(\w+)\s+\g1/;    # match doubled words of arbitrary	length
	   $year =~ /^\d{2,4}$/;  # make sure year is at least 2 but not more
				  # than 4 digits
	   $year =~ /^\d{ 4 }$|^\d{2}$/; # better match; throw out 3 digit dates

       These quantifiers will try to match as much of the string as possible,
       while still allowing the	regex to match.	 So we have

	   $x =	'the cat in the	hat';
	   $x =~ /^(.*)(at)(.*)$/; # matches,
				   # $1	= 'the cat in the h'
				   # $2	= 'at'
				   # $3	= ''   (0 matches)

       The first quantifier ".*" grabs as much of the string as	possible while
       still having the	regex match. The second	quantifier ".*"	has no string
       left to it, so it matches 0 times.

   More	matching
       There are a few more things you might want to know about	matching
       operators.  The global modifier "/g" allows the matching	operator to
       match within a string as	many times as possible.	 In scalar context,
       successive matches against a string will	have "/g" jump from match to
       match, keeping track of position	in the string as it goes along.	 You
       can get or set the position with	the "pos()" function.  For example,

	   $x =	"cat dog house"; # 3 words
	   while ($x =~	/(\w+)/g) {
	       print "Word is $1, ends at position ", pos $x, "\n";
	   }

       prints

	   Word	is cat,	ends at	position 3
	   Word	is dog,	ends at	position 7
	   Word	is house, ends at position 13

       A failed	match or changing the target string resets the position.  If
       you don't want the position reset after failure to match, add the "/c",
       as in "/regex/gc".

       In list context,	"/g" returns a list of matched groupings, or if	there
       are no groupings, a list	of matches to the whole	regex.	So

	   @words = ($x	=~ /(\w+)/g);  # matches,
				       # $word[0] = 'cat'
				       # $word[1] = 'dog'
				       # $word[2] = 'house'

   Search and replace
       Search and replace is performed using "s/regex/replacement/modifiers".
       The "replacement" is a Perl double-quoted string	that replaces in the
       string whatever is matched with the "regex".  The operator "=~" is also
       used here to associate a	string with "s///".  If	matching against $_,
       the "$_ =~" can be dropped.  If there is	a match, "s///"	returns	the
       number of substitutions made; otherwise it returns false.  Here are a
       few examples:

	   $x =	"Time to feed the cat!";
	   $x =~ s/cat/hacker/;	  # $x contains	"Time to feed the hacker!"
	   $y =	"'quoted words'";
	   $y =~ s/^'(.*)'$/$1/;  # strip single quotes,
				  # $y contains	"quoted	words"

       With the	"s///" operator, the matched variables $1, $2, etc.  are
       immediately available for use in	the replacement	expression. With the
       global modifier,	"s///g"	will search and	replace	all occurrences	of the
       regex in	the string:

	   $x =	"I batted 4 for	4";
	   $x =~ s/4/four/;   #	$x contains "I batted four for 4"
	   $x =	"I batted 4 for	4";
	   $x =~ s/4/four/g;  #	$x contains "I batted four for four"

       The non-destructive modifier "s///r" causes the result of the
       substitution to be returned instead of modifying	$_ (or whatever
       variable	the substitute was bound to with "=~"):

	   $x =	"I like	dogs.";
	   $y =	$x =~ s/dogs/cats/r;
	   print "$x $y\n"; # prints "I	like dogs. I like cats."

	   $x =	"Cats are great.";
	   print $x =~ s/Cats/Dogs/r =~	s/Dogs/Frogs/r =~
	       s/Frogs/Hedgehogs/r, "\n";
	   # prints "Hedgehogs are great."

	   @foo	= map {	s/[a-z]/X/r } qw(a b c 1 2 3);
	   # @foo is now qw(X X	X 1 2 3)

       The evaluation modifier "s///e" wraps an	"eval{...}" around the
       replacement string and the evaluated result is substituted for the
       matched substring.  Some	examples:

	   # reverse all the words in a	string
	   $x =	"the cat in the	hat";
	   $x =~ s/(\w+)/reverse $1/ge;	  # $x contains	"eht tac ni eht	tah"

	   # convert percentage	to decimal
	   $x =	"A 39% hit rate";
	   $x =~ s!(\d+)%!$1/100!e;	  # $x contains	"A 0.39	hit rate"

       The last	example	shows that "s///" can use other	delimiters, such as
       "s!!!" and "s{}{}", and even "s{}//".  If single	quotes are used
       "s'''", then the	regex and replacement are treated as single-quoted
       strings.

   The split operator
       "split /regex/, string" splits "string" into a list of substrings and
       returns that list.  The regex determines	the character sequence that
       "string"	is split with respect to.  For example,	to split a string into
       words, use

	   $x =	"Calvin	and Hobbes";
	   @word = split /\s+/,	$x;  # $word[0]	= 'Calvin'
				     # $word[1]	= 'and'
				     # $word[2]	= 'Hobbes'

       To extract a comma-delimited list of numbers, use

	   $x =	"1.618,2.718,	3.142";
	   @const = split /,\s*/, $x;  # $const[0] = '1.618'
				       # $const[1] = '2.718'
				       # $const[2] = '3.142'

       If the empty regex "//" is used,	the string is split into individual
       characters.  If the regex has groupings,	then the list produced
       contains	the matched substrings from the	groupings as well:

	   $x =	"/usr/bin";
	   @parts = split m!(/)!, $x;  # $parts[0] = ''
				       # $parts[1] = '/'
				       # $parts[2] = 'usr'
				       # $parts[3] = '/'
				       # $parts[4] = 'bin'

       Since the first character of $x matched the regex, "split" prepended an
       empty initial element to	the list.

   "use	re 'strict'"
       New in v5.22, this applies stricter rules than otherwise	when compiling
       regular expression patterns.  It	can find things	that, while legal, may
       not be what you intended.

       See 'strict' in re.

BUGS
       None.

SEE ALSO
       This is just a quick start guide.  For a	more in-depth tutorial on
       regexes,	see perlretut and for the reference page, see perlre.

AUTHOR AND COPYRIGHT
       Copyright (c) 2000 Mark Kvale All rights	reserved.

       This document may be distributed	under the same terms as	Perl itself.

   Acknowledgments
       The author would	like to	thank Mark-Jason Dominus, Tom Christiansen,
       Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful
       comments.

perl v5.35.11			  2022-03-27			PERLREQUICK(1)

NAME | DESCRIPTION | The Guide | BUGS | SEE ALSO | AUTHOR AND COPYRIGHT

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