programming-examples/perl/Regular_Expression/Anchor.pl

# The ^ and $ Pattern Anchors!
^ and $ matches only at the beginning or the end of a string. 

/^def/ matches def only if these are the first three characters in the string. 
/def$/ matches def only if these are the last three characters in the string. 

combine ^ and $ to force matching of the entire string. 
/^def$/ matches only if the string is def. 





# Check beginning!

#!/usr/bin/perl -w

foreach $patt (@ARGV) {


    # Check beginning.
    if ($patt =~ /^perl/) {
        print "\tFound perl at start in \"$patt\".\n";
    }
}




#Check beginning and end!

#!/usr/bin/perl -w

foreach $patt (@ARGV) {


    # Check beginning and end.
    if ($patt =~ /^perl$/) {
        print "\tFound only perl in \"$patt\".\n";
    }

}




#Check end!

#!/usr/bin/perl -w

foreach $patt (@ARGV) {

    
    # Check end.
    if ($patt =~ /perl$/) {
        print "\tFound perl at end in \"$patt\".\n";
    }
}





#End of line anchor!

while(<DATA>){
    print if /10$/;
}
__DATA__
    1.10
    .5
    555.10
    4.01
    .501
    601






    #A negative look ahead!

while(<DATA>){
   print if/^\w+\s(?![BC])/;
}
__DATA__
    ABC
    CBC




    #A negative look behind!

while(<DATA>){
    print if /(?<!B) B[a-z]*/;
}
__DATA__
    Betty
    CBC
    ABC




    #A positive look ahead!   

$string="I love chocolate.";
$string =~ s/chocolate(?= ice)/vanilla/;
print "$string\n";

$string="this is a test.";
$string =~ s/this(?=is)/test/g;
print "$string\n";





#A positive look behind!

$string="chocolate/cake/milk/ice cream.";
$string =~ s/(?<= chocolate) milk/ candy/;
print "$string\n";

$string="this is a test.";
$string =~ s/(?<=a test) is/ is not/;
print "$string\n";

   
    
    
 #Anchoring Metacharacters!    

Metacharacter   What It Matches
^               Matches to beginning of line or beginning of string
$               Matches to end of line or end of a string
\A              Matches the beginning of the string only
\Z              Matches the end of the string or line
\z              Matches the end of string only
\G              Matches where previous m//g left off
\b              Matches a word boundary (when not inside [ ])
\B              Matches a nonword boundary

   
    
    
  #Beginning of line anchor!
    
while(<DATA>){

   print if /^[JK]/;
}
__DATA__
    Mark 
    Mary
    Jack
    Kate




   #Clustering and anchors!

while(<DATA>){
    # print if /^A|B/;
    print if /^(S|B)/;
}
__DATA__
    S B
    B B
    A C
    N C
    J D
    K E

   
  


  #Beginning and end of word anchors!

while(<DATA>){
   print if /\bJon\b/;
}
__DATA__
    Jonathan 
    Jason
    Mary
    Mark

   
    
 #Get begin of the line!

$_ = "This text\nhas multiple lines.";
s/^/BOL/g;
s/$/EOL/g;
print;

   
    
    
#Searching from the beginning and the end!

You can search for a pattern at a specified location, such as the beginning or end of the string.

Pattern     Interpretation
/^a/        Match against a only at beginning of string.
/a$/        Match against a only at end of string.
/a\b/       Match a at end of word.
/a\B/       Match a not at end of word.



/$a/ means to match the value of $a.
/a$/ matches against an a at the end of the string. 
/$a$/ matches against the value of the variable $a at the end of the string.
The ^ character acts differently depending on whether it is inside a square bracket or not. 
/^a/ looks for a at the start of the string. 
/[^a]/ will return true if there is any character other than a anywhere in the word. 

   
    
    
 #Shouldn't start a sentence with a period!

$line = ".Hello!";

if ($line =~ m/\A\./) {
    print "Shouldn't start a sentence with a period!";
}

   
    
    
 #Word-Boundary Pattern Anchors!

\b and \B, specify whether a matched pattern must be on a word boundary or inside a word boundary. 
The \b pattern anchor specifies that the pattern must be on a word boundary. 

/\bdef/ matches only if def is the beginning of a word. 

\b to indicate the end of a word. 
/def\b/ matches def and abcdef

/\bdef\b/ matches only the word def.