A.13 Binding Operators
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: 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; 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. |