What do 'lazy' and 'greedy' mean in the context of regular expressions?

regex regex-greedy non-greedy
ajsie picture ajsie · Feb 20, 2010 · Viewed 263.9k times · Source

Could someone explain these two terms in an understandable way?

Answer

Sampson picture Sampson · Feb 20, 2010

Greedy will consume as much as possible. From http://www.regular-expressions.info/repeat.html we see the example of trying to match HTML tags with <.+>. Suppose you have the following:

<em>Hello World</em>

You may think that <.+> (. means any non newline character and + means one or more) would only match the <em> and the </em>, when in reality it will be very greedy, and go from the first < to the last >. This means it will match <em>Hello World</em> instead of what you wanted.

Making it lazy (<.+?>) will prevent this. By adding the ? after the +, we tell it to repeat as few times as possible, so the first > it comes across, is where we want to stop the matching.

I'd encourage you to download RegExr, a great tool that will help you explore Regular Expressions - I use it all the time.

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