Perl | Anchors in Regex - GeeksforGeeks

Anchors in Perl Regex do not match any character at all. Instead, they match a particular position as before, after, or between the characters. These are used to check not the string but its positional boundaries.
Following are the respective anchors in Perl Regex:

'^' '$', '\b', '\A', '\Z', '\z', '\G', '\p{....}', '\P{....}', '[:class:]'

^ or \A: It matches the pattern at the beginning of the string.

Syntax: (/^pattern/, /\Apattern/).

Example:

#!/usr/bin/perl 
$str = "guardians of the galaxy"; 
  
# prints the pattern as it is 
# starting with 'guardians' 
print "$&\n" if($str =~ /^guardians/); 
  
# prints the pattern 'gua' 
print "$&\n" if($str =~ /\Agua/); 
  
# prints nothing because  
# the 0th position doesn't start with 'a' 
print "$&" if($str =~ /^ans/) 

Output:

guardians
gua

$ or \z: It matches the pattern at the end of the string.

Syntax: (/pattern$/, /pattern\z/).

Example:

#!/usr/bin/perl 
$str = "guardians of the galaxy"; 
  
# prints nothing as it is not  
# ending with 'guardians' 
print "$&\n" if($str =~ /guardians$/); 
  
# prints the pattern 'y' 
print "$&\n" if($str =~ /y\z/); 
  
# prints the pattern as it is  
# ending with 'galaxy' 
print "$&" if($str =~ /galaxy$/) 

Output:

y
galaxy

\b: It matches at the word boundary of the string from \w to \W. In precise, it either gets a match to beginning or end of the string if it is a word or to a word character or a non-word character.

Syntax: (/\bpattern\b/).

Example:

#!/usr/bin/perl 
$str = "guardians-of-the-galaxy"; 
  
# prints '-galaxy' as it forms 
# a word even with '-'. 
print "$&\n" if($str =~ /\b-galaxy\b/); 
  
# prints '-guardians' as it forms  
# a word even with '-'. 
print "$&\n" if($str =~ /\bguardians-\b/); 
  
# prints nothing as it is bounded 
# with a character 't'. 
print "$&" if($str =~ /\be-galaxy\b/); 
  
# prints 'guardians-of-the-galaxy' as it 
# is bounded with the beginning and end. 
print "$&" if($str =~ /\bguardians-of-the-galaxy\b/); 

Output:

-galaxy
guardians-
guardians-of-the-galaxy

\Z: It matches at the ending of the string or before the newline. ‘\z‘ and ‘\Z‘ both differ from $ in that they are not affected by the /m “multiline” flag, which allows $ to match at the end of any line.

#!/usr/bin/perl 
  
# Prints one due to m// 
print "one\n" if ('galaxy' =~ m/galaxy\z/); 
  
# Prints two due to m// 
print "two\n" if('galaxy' =~ m/galaxy\Z/); 
  
# Prints three due to /Z  
# as it forms a newline 
print "three\n" if ("galaxy\n" =~ m/galaxy\Z/); 
  
# Prints four due to m// as  
# the line ended \z gets affected 
print "four\n" if ("galaxy\n" =~ m/galaxy\n\z/); 
  
# Prints five as it forms a new line 
print "five\n" if("galaxy\n" =~ m/galaxy\n\Z/); 
  
# Due to the "" it forms a newline and  
# \z doesn't get effected. Prints nothing 
print "six" if("galaxy\n" =~ m/galaxy\z/); 

Output:

one
two
three
four
five

\G: It matches at the specified position. If a pattern’s length is 5 then it starts from the start of the string till 5 positions, if the pattern is valid then it is forced to check the string from 6th position onwards, moves forward in this fashion till pattern not valid or end of the string.

#!/usr/bin/perl 
$str = "galaxy8222as"; 
  
# prints until the pattern is valid 
print "one: $& " while($str =~ /\G[a-z]{2}/gc); 
print "\n"; 
  
# prints until the pattern is valid 
print "two: $& " while("1122a44" =~ /\G\d\d/gc); 
print "\n"; 
  
# Take the string as a new value and  
# searches from the start to false 
print "three: $& " while("galaxy8222as" =~ /\G\w{2}/gc); 
print "four: $& " while($str =~ /\G[a-z]{2}/gc); 
  
# Take the false position of the  
# above string and searches from there 
# Prints if the pattern is valid from that position  
# onwards(prints nothing). As it is false  
# it stays at the same position as before. 
print "\n"; 
print "five: $& " while($str =~ /\G\w{2}/gc); 

Output:

one: ga one: la one: xy 
two: 11 two: 22 
three: ga three: la three: xy three: 82 three: 22 three: as 
five: 82 five: 22 five: as

\p{…} and \P{…}: \p{…} matches Unicode character class like IsLower, IsAlpha, etc. whereas \P{….} is the complement of Unicode character class.

#!/usr/bin/perl 
  
# unicode class is the pattern to match 
print "$&" while("guardians!@#%^*123" =~ /\p{isalpha}/gc); 
print "\n"; 
  
# unicode class is the pattern to match 
print "$&" while("guardians!@#%^&*123" =~ /\p{isalnum}/gc); 
print "\n"; 
  
# here L matches the alphabets where \P is the complement 
print "$&" while("guardians!@#%^&*123" =~ /\P{L}/gc); 
print "\n"; 
  
# here L matches the alphabets where \p is non-complement 
print "$&" while("guardians!@#%^&*123" =~ /\p{L}/gc); 

Output:

guardians
guardians123
!@#%^&*123
guardians

[:class:]: POSIX Character Classes like digit, lower, ascii, etc.

Syntax: (/[[:class:]]/)

POSIX character classes are as follows:

alpha, alnum, ascii, blank, cntrl, digit, graph, lower, punct, space, upper, xdigit, word

#!/usr/bin/perl 
  
# prints only alphabets 
print "$&" while('guardians!@#%^&*123' =~ /[[:alpha:]]/gc); 
print "\n"; 
  
# prints characters and digits 
print "$&" while("guardians!@#%^&*123" =~ /[[:alnum:]]/gc); 
print "\n"; 
  
# prints only digits 
print "$&" while("guardians!@#%^&*123" =~ /[[:digit:]]/gc); 
print "\n"; 
  
# prints anything except space " ". 
print "$&" while("guardians!@#%^& 123\n" =~ /[[:graph:]]/gc); 
print "\n"; 
  
# prints the 1 as it gets matched to  
# space " " or horizontal tab. 
print "1" while("guardians!@#%^& 123\n" =~ /[[:blank:]]/gc); 
print "\n"; 
  
# prints lowercase characters 
print "$&" while("Guardians!@#%^& 123\n" =~ /[[:lower:]]/gc); 
print "\n"; 
  
# prints all ascii characters 
print "$&" while("guardians!@#%^& 123\n" =~ /[[:ascii:]]/gc); 

Output:

guardians
guardians123
123
guardians!@#%^&123
1
uardians
guardians!@#%^& 123

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