Non-greedy regex acts greedily

Question

Here's a simple example:

Text: <input name="zzz" value="18754" type="hidden"><input name="zzz" value="18311" type="hidden"><input name="zzz" value="17138" type="hidden">

Regex: /<input.*?value="(18754|17138)".*?>/

When matches are replaced by an empty string, the result is an empty string. I expected the middle <input> to remain since I am using non-greedy matching (.*?). Anyone could explain why it is removed?

What does the following mean: *"When matches are replaced by an empty string"*? — Bart Kiers, Jan 23 '12 at 12:31
Your regex matches the entire input tag, so the engine picks up the entire input tag and replaces that with an empty string. What happens then is that your three input tags are completely obliterated. — BoltClock, Jan 23 '12 at 12:32
@BoltClock, no, the pattern matches two times: **1:** `` and **2:** ``. Or did I misunderstand? — Bart Kiers, Jan 23 '12 at 12:33
@BoltClock: but the regex should not match the middle ``. I don't get why it does. — Ree, Jan 23 '12 at 12:34
This immediately reminded me about [this answer](http://stackoverflow.com/a/1732454/1076353). — RCE, Jan 23 '12 at 12:47

score 5 · Answer 1 · answered Jan 23 '12 at 12:32

There are two matches:

<input name="zzz" value="18754" type="hidden">
<input name="zzz" value="18311" type="hidden"><input name="zzz" value="17138" type="hidden">

In the second case, the first .*? matches name="zzz" value="18311" type="hidden"><input name="zzz". It's a match and it's non-greedy.

score 2 · Answer 2 · answered Jan 23 '12 at 12:38

2

aix already explained, why it does match the middle part.

To avoid this behaviour, get rid of the .*?, instead try this:

/<input[^>]*value="(18754|17138)"[^>]*>/

See it here on Regexr

Instead of matching any character, match any, but ">"

answered Jan 23 '12 at 12:38

stema

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score 0 · Answer 3 · edited May 23 '17 at 12:27

aiz's answer is correct -- the second match includes the 2nd and 3rd input tags.

One possible fix for your regex would be to change . to [^>], like this:

/<input[^>]*?value="(18754|17138)"[^>]*?>/

That will cause it to match any character except >. But that has the obvious problem of breaking whenever > shows up inside a quoted literal. As everyone always says: Regexes aren't designed to work on HTML. Don't use them unless you have no other choice.

Non-greedy regex acts greedily

3 Answers3

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