April 6, 2007

• Regular Expressions: Multiline vs. Singleline

  This post is really just for my own reference.  Writing regular expressions can be a complicated experience with a lot of guess and check.  The one thing that always confuses me is the definition of Multiline vs. Singleline.  They have nothing to do with what they intuitively seem to do; they aren’t even related to each other!  And every time I encounter these, I go scrambling across the internet trying to remember what they do.  So for posterity, I wanted to set the record straight on these two options:

  Multiline

  Changes the definition of ^ and $ so that they match at individual lines of the input string (instead of the beginning and end of the entire input string).  For example if you have the string: With a taste of your lips I’m on a ride You’re toxic I’m slipping under Multiline OFF will match ^ to the beginning of “With” and $ to the end of “under”, while Multiline ON will match ^ and $ to the beginning and end of each individual line (so there are 4 possible matches instead of 1).

  Singleline

  Changes the definition of the period character (.) so that it matches all characters, including newlines.  Taking the example text from above, a period with Singline OFF will match only to the end of the first line, whereas Singleline ON will match the entire text (i.e. it pretends the input is a single line).

  So Multiline is relevant only when using ^ and $, while Singleline is relevant only when using the period character.  They are unecessary in any other situations, and they don’t interact with each other.

  If you want to turn regular epxressions from a chore into something fun, I highly recommend Jeffrey Friedl’s Mastering Regular Expressions.  Not only is it the definitive book on practical regular expressions, its one of the best programming books I’ve ever read.

  • 12:02 pm
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