B.12 Binding Operators

Binding operators are used for pattern matching, substitution, and transliteration on strings. They are used in conjunction with regular expressions that specify the patterns. Here's an example:

'ACGTACGTACGTACGT' =~ /CTA/

The pattern is the string CTA, enclosed by forward slashes //. The string binding operator is =~; it tells the program which string to search, returning true if the pattern appears in the string.

Another string binding operator is !~, which returns true if the pattern isn't in the string:

'ACGTACGTACGTACGT' !~ /CTA/

This is equivalent to:

not 'ACGTACGTACGTACGT' =~ /CTA/

You can substitute one pattern for another using the string binding operator. In the next example, s/thine/nine/ is the substitution command, which substitutes the first occurrence of thine with the string nine:

$poor_richard = 'A stitch in time saves thine.';
$poor_richard =~ s/thine/nine/;
print $poor_richard;

This produces the output:

A stitch in time saves nine.

Finally, the transliteration (or translate) operator tr substitutes characters in a string. It has several uses, but the two uses I've covered are first, to change bases to their complements A T, C A:

$DNA = 'ACGTTTAA';
$DNA =~ tr/ACGT/TGCA/;

This produces the value:

TGCAAATT

Second, the tr operator counts the number of a particular character in a string, as in this example which counts the number of Gs in a string of DNA sequence data:

$DNA = 'ACGTTTAA';
$count = ($DNA =~ tr/A//);
print $count;

This produces the value 3. This shows that a pattern match can return a count of the number of translations made in a string, which is then assigned to the variable $count.