Regular Expressions

Anchoring a pattern to the beginning or end of a string

To force matching of your pattern to the beginning of the string, write a caret ^ as the first character of the regular expression.

To force the matching to the end of the string, write a dollar sign $ as the last character of the regular expression.

Example 1

We have previously learned how to extract desired lines from a Swiss-Prot entry, using the substring function.

Now let us see how to do that using a regular expression matching (though it is less efficient in this case).

To print the "description" line, which starts with DE, we write: 

#!/usr/local/bin/perl

my $sp_file = "sources/sp_entry";

open (SP, $sp_file) || die "cannot open \"$sp_file\": $!";

while (my $line = <SP>) {
   if ($line =~ /^DE/) {
      print $line;
   }
}
Result: 
DE   MUSCARINIC ACETYLCHOLINE RECEPTOR M1.
Note: if we omitted the caret from the regular expression: 
if ($line =~ /DE/)
We would have got: 
DE   MUSCARINIC ACETYLCHOLINE RECEPTOR M1.
RA   ARDEN J.R., NAGATA O., SHOCKLEY M.S., PHILIP M., LAMEH J., SADEE W.;
CC       CELLULAR RESPONSES, INCLUDING INHIBITION OF ADENYLATE CYCLASE,
CC       BREAKDOWN OF PHOSPHOINOSITIDES & MODULATION OF POTASSIUM CHANNELS
     SERSQPGAEG SPETPPGRCC RCCRAPRLLQ AYSWKEEEEE DEGSMESLTS SEGEEPGSEV

Example 2

To match all file names or paths that have the .pl extension, write: 
if ($file =~ /\.pl$/) {  }

# Note: there is a backslash before the dot, since we mean a literal
# dot and not "any character except newline" (see here).

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