Question

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Regex question mark

I am trying to figure out how to parse out the text inside the parenthesis but between the single quotes in a statement. For example, if I have the following statement:

(I have a 'cat', 'hat');

I want the result to be

cat
hat

I managed to figure it out by experimenting with the different metacharacters defined in wikipedia (http://en.wikipedia.org/wiki/Regular_expression) however I still have trouble understanding why it works.

I’ve tried this : \'(.*)\'

My understanding of this regex: I want to get the characters in between the single quotes ' and these characters matches any single character . zero or more times *.

This resulted in:

cat', 'hat

After playing around with a bunch of regex I finally ended up with this by accident: \'(.*?)\'

This resulted in:

cat
hat

Why does this work? (In particular I don’t understand how the ‘?’ works.)

Answer 1

The default behavior of the regular expression to make the longest possible match in the string. This is referred to as being “greedy.”

You are correct that ? normally just means to match the preceding item (e.g., character) one or more times, but *? is a special case called “lazy star” that switches the regular expression evaluator into a “lazy” mode. In this mode, the evaluator first tries to skip the preceding item (and complete a match without it), before then “going back” for it.

The net result is just what you’ve observed: it will match shorter strings that satisfy the search criteria instead of (the default) whereby it would just find the longest possible match.

A handy resource for testing out regular expressions is here, and a nice description of the various options, including lazy star, is here.