Part of what makes Perl such a useful language is its powerful string-matching and handling capabilities. Regular expressions are basically patterns a programmer can compare a string of text to. Matching a regular expression with a string of text either returns true or false. The two main pattern matching operators are m// and s//. These are the matching and substitution operators respectively. Another function that makes use of regular expressions is split The matching operator m// is normally written //. Perl allows you to change the delimiters to something besides /. If you don't change the delimiters from /, you can use // instead of m//. Now for a quick example of m//:
while(<>){ if(/the/){ #does $_ contain the print "Your line of text contains the word 'the'\n"; } }

Now for a quick example of s///:
@machines_os=("OpenBSD","Windows","Linux", "Windows"); foreach(@machines_os){ s/Windows/Linux/; }

This function goes through each of the items in @machine_os. If any of them contain Windows, the thing between the first set of //. It is replaced with Linux the string between the second and third /'s. You can see why you've gotta love Perl. Instead of 1 OpenBSD machine, 2 Windows machines and a Linux box, I now have 1 OpenBSD box and 3 Linux machines. At least that is what @machines_os now contains.

Now on to Quantifiers in regular expressions.