Perl for Web Site Management

Chapter 7.  Regular Expressions Demystified

So far, we’ve been treating regular expressions like unexploded bombs: dangerous, complicated things to be approached with extreme caution. It’s time now to learn how they really work so that you can take them apart and put them back together and understand what’s going on. It’s not terribly difficult once you get the hang of it, and there’s a big payoff once you do, but there’s a fair amount of detail to learn along the way. We’ll start with the outside of the expression (the delimiters and trailing modifiers), then spend most of our time looking at the search pattern syntax itself.

Delimiters

At the beginning and end of every regular expression is a delimiter (and if you’re talking about a substitution variety of regular expression, there will be one or two more in the middle). The delimiter is a special character that indicates the boundaries of the expression. By default, the delimiter is a forward slash (/), but as you learned previously, you can choose another delimiter if you like in order to make the expression more readable.

As you also saw previously, if you choose as your custom delimiter an opening parenthesis (or one of the other paired delimiters: curly braces, square brackets, or angle brackets), the closing delimiter will be the corresponding closing character from that pair. Additionally, if you do this with a substitution variety of regular expression, you won’t have a single delimiter in the middle of the expression, ...