Regular expressions, also known as "regex" by the geek community, are a powerful tool used in pattern-matching and substitution. They are commonly associated with almost all *NIX-based tools, including editors like vi, scripting languages like Perl and PHP, and shell programs like awk and sed.
A regular expression lets you build patterns using a set of special characters; these patterns can then be compared with text in a file, data entered into an application, or input from a form filled up by users on a Web site. Depending on whether or not there's a match, appropriate action can be taken, and appropriate program code executed.
For example, one of the most common applications of regular expressions is to check whether or not a user's email address, as entered into an online form, is in the correct format; if it is, the form is processed, whereas if it's not, a warning message pops up asking the user to correct the error. Regular expressions thus play an important role in the decision-making routines of Web applications - although, as you'll see, they can also be used to great effect in complex find-and-replace operations.
A regular expression usually looks something like this:
All this does is match the pattern "matrix" in the text it's applied to. Like many other things in life, it's simpler to get your mind around the pattern than the concept. Then again, that's neither here nor there.
How about something a little more complex? Try this:
This would match the words "matting" and "mattress", but not "matrix". Why? Because the "+" character is used to match one or more occurrence of the preceding character - in the example above, the characters "ma" followed by one or more occurrence of the letter "t".
Similar to the "+" meta-character (that's the official term), we have "*" and "?". These are used to match zero or more occurrences of the preceding character, and zero or one occurrence of the preceding character, respectively. So,
would match "easy", "egocentric" and "egg", while
would match "Winnie", "Wimpy" "Wilson" and "William", though not "Wendy" or "Wolf". In case all this seems a little too imprecise, you can also specify a range for the number of matches. For example, the regular expression
would match "jimmy" and "jimmmmmy!", but not "jim". The numbers in the curly braces represent the lower and upper values of the range to match; you can leave out the upper limit for an open-ended range match.