Regular Expressions: "Does not start with...."

[This was originally posted at http://timstall.dotnetdevelopersjournal.com/regular_expressions_does_not_start_with.htm]

 

A coworker recently asked me how to have a regular expression check if something does not start with a certain value. For example, how to tell if an input value does not start with 'abc'.

Note that it's very easy to see if something does start with a certain value, just use the anchor-to-first-start char '^'. For example, this regex will check for all strings that start with 'abc' (followed by any \w character(s)):

    ^abc\w+

To see that something does not start with a given value, use the Grouping Construct 'Zero-width negative lookahead assertion":

    ^(?!abc)\w+

This would handle the following cases:

Pass - none of these start with 'abc':	Fail - all of these start with 'abc':
defg ab xyz 11999	abc abcdef

Note that there are four similar grouping constructs based on the combos of Positive/Negative - Lookahead/Lookbehind

• Positive Lookahead
• Negative Lookahead
• Positive Lookbehind
• Negative Lookbehind

You can download a free regex editor from MVP Roy Osherove