Regexp stands for regular expressions. Regular expressions are what made languages like Awk and Perl notorious. Regular expressions are expressions equating a pattern string. To write a regular expression in Perl on must insert it between slashes (/regexp/)For example:
means all the alphabet lowercase letters.
stands for all the digits from 0 to 4 inclusive.
stands for all the sequences of 2, 5 or 7 digits.
The interesting thing is that you can compose regexps into complex patterns. For example the regexp
describes a pattern that will match, for example, the strings B:1.HelloSir or 'A:0.Good. But it will not match a:2.darn because the first letter must be uppercase.
Bounding a whole word: