.NET Regular Expressions: how to use RegexOptions.IgnorePatternWhitspace [Ryan Byington]

The IgnorePatternWhitespace option tells the Regex parser to ignore any spaces or tabs in your expression except if it is in a character class(ie [ ]). At first this may not seem all that useful but it really can increase the readability of a regular expression. Plus you can add comments to your expression.


For example a customer complained that the following regular expression that is suppose to match email addresses was taking to long:




Maybe I am just not very good at visualizing regular expressions but I have trouble seeing what this regular expression does when it is in this format. The first thing I do is convert it to a format that I can read like the following:




            [0-9a-zA-Z] #Verify the email address starts with a valid character












            [a-zA-Z]{2,9} #Match to com, org, uk, etc




This equivalent to the first expression as long as the RegexOptions.IgnorePatternWhitespace option is used. The only trick here is that to match a space you must either use [ ] or \x20. If you are curious what was wrong with this expression the ‘.’ in [-.\w]* needed to be escaped.


It is possible to write an entire c# program on a single line with no indenting but no one in their right mind would do this because it would be completely unreadable. So why do this with your regular expressions?

Comments (4)

  1. Anonymous says:

    Hey– do you know of any apps that will "pretty format" a regex? That’d be cool. Because 99.9% of the time they’re presented as one giant line.

    Canonical example (WARNING: I cannot be held responsible for any bleeding of the eyes that may occur after viewing this regex)


  2. Anonymous says:

    All right… I will! I’m continually amazed how useful regular expressions are in my daily coding. I’m still working on the MhtBuilder refactoring, and I needed a function to convert all URLs in a HTML string from relative to…

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