Programming Perl, 4th Edition

Regular Expressions

Regular expressions (a.k.a. regexes, regexps, or REs) are used by many search programs such as grep and findstr, text-munging programs like sed and awk, and editors like vi and emacs. A regular expression is a way of describing a set of strings without having to list all the strings in your set.^[31] Many other computer languages incorporate regular expressions (some of them even advertise “Perl5 regular expressions”!), but none of these languages integrates regular expressions into the language the way Perl does. Regular expressions are used several ways in Perl. First and foremost, they’re used in conditionals to determine whether a string matches a particular pattern, because in a Boolean context they return true and false. So when you see something that looks like /foo/ in a conditional, you know you’re looking at an ordinary pattern-matching operator:

if (/Windows 7/) { print "Time to upgrade?\n" }

Second, if you can locate patterns within a string, you can replace them with something else. So when you see something that looks like s/foo/bar/, you know it’s asking Perl to substitute “bar” for “foo”, if possible. We call that the substitution operator. It also happens to return true or false depending on whether it succeeded, but usually it’s evaluated for its side effect:

s/IBM/lenovo/;

Finally, patterns can specify not only where something is, but also where it isn’t. So the split operator uses a regular expression to specify where the data isn’t. That is, the ...