String Substitution

String substitution allows you to replace a pattern or character range with another one using the s/// and tr/// functions.

The s/// Function

s/// has two parts: the regular expression and the string to replace it with: s/expression/replacement/.

$h = "Who's afraid of the big bad wolf?"; $i = "He had a wife."; $h =~ s/w.+f/goat/; # yields "Who's afraid of the big bad goat?" $i =~ s/w.+f/goat/; # yields "He had a goate."

If you extract pattern matches, you can use them in the replacement part of the substitution:

$h = "Who's afraid of the big bad wolf?"; $h =~ s/(\w+) (\w+) wolf/$2 $1 wolf/; # yields "Who's afraid of the bad big wolf?"

Default Substitution Variable

If you don't bind a variable with =~, then s/// operates on $_ just as the match does.

Using a Variable in the Substitution Part

Yes you can:

$h = "Who's afraid of the big bad wolf?"; $animal = 'hyena'; $h =~ s/(\w+) (\w+) wolf/$2 $1 $animal/; # yields "Who's afraid of the bad big hyena?"

Using Different Delimiters

The s/// function can use alternative delimiters, including parentheses and bracket pairs. For example:

$h = "Who's afraid of the big bad wolf?"; $h =~ s!(\w+) (\w+) wolf!$2 $1 wolf!; # using ! as delimiter $h =~ s{(\w+) (\w+) wolf}{$2 $1 wolf}; # using {} as delimiter

Translating Character Ranges

The tr/// function allows you to translate one set of characters into another. Specify the source set in the first part of the function, and the destination set in the second part:

$h = "Who's afraid of the big bad wolf?"; $h =~ tr/ao/AO/; # yields "WhO's AfrAid Of the big bAd wOlf?";

Like s///, the tr/// function operates on $_ if not otherwise specified.

tr/// returns the number of characters transformed, which is sometimes handy for counting the number of a particular character without actually changing the string.

This example counts N's in a series of DNA sequences:

Code:

  
  while (<>) {
    chomp;   # assume one sequence per line
    my $count = tr/Nn/Nn/;
    print "Sequence $_ contains $count Ns\n";
  }

Output:

(~) 50% count_Ns.pl sequence_list.txt
Sequence 1 contains 0 Ns
Sequence 2 contains 3 Ns
Sequence 3 contains 1 Ns
Sequence 4 contains 0 Ns
...

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