Binding operators are used for pattern matching, substitution, and transliteration on strings. They are used with regular expressions that specify the patterns:
'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.
!~ is another string binding operator; it 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 G, G C, and T A:
$DNA = 'ACGTTTAA'; $DNA =~ tr/ACGT/TGCA/;
This produces the value:
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; it 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.