[Regex-tut] A gentle introduction

[wtd]

Prerequisites

Install Ruby, if you haven't already.

What does a regular expression look like?

Ruby gives us a very light syntax for defining regular expressions. A simple regular expression to match the string:

code:

"Hello, world!"

Is:

code:

/Hello, world!/

How do you actually test to see if there's a match?

The =~ operator permits us to test a string against a regular expression, or vice versa. The =~ operator is commutative.

code:

if "Hello, world!" =~ /Hello, world!/    puts "It matched!" else    puts "It didn't match." end

That's all well and good, but...

... so far we've only been dealing with a static pattern. Or have we?

A regular expression such as:

code:

/Hello, world!/

Doesn't have to match the entire string. In fact...

code:

"foo Hello, world! bar"

Would match that regular expression. Now, maybe that's good enough. But maybe it isn't. To match from the beginning of a string, use:

code:

/^Hello, world!/

The caret at the beginning of the regex says, "start searching at the beginning of the string." To indicate the end of the string:

code:

/Hello, world!$/

And of course, combine them:

code:

/^Hello, world!$/

And the pattern will only match:

code:

"Hello, world!"