Regular expression in java to match XML

Question

I'm locked in a regex problem. I must put "<strong>" and "</strong>" tags at the sides of some String, along a larger String. For example, if I have:

"This is a test, and test word appears two times"

And the String selected is "test", it will remains:

"This is a <_strong>test<_/strong>, and <_strong>test<_/strong> word appears two times"

At first, I think in use regex functions combined with "ReplacedAll". The problem comes when there are <_strong> tags in the larger String, some like that:

"This is a test, and <_strong>test word<_/strong> appears two times"

It will remain something like that:

"This is a <_strong>test<_/strong>, and <_strong><_strong>test<_/strong> word<_/strong> appears two times"

The idea is for find a regular expression that modify the string "test" only if there isn't between <_strong> tags. But I'm not able for find it.

http://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags/1732454#1732454 — chrylis -cautiouslyoptimistic-, Sep 25 '13 at 11:58
@chrylis http://meta.stackexchange.com/questions/182189/please-stop-linking-to-the-zalgo-anti-cthulhu-regex-rant — sp00m, Sep 25 '13 at 12:20
@sp00m Fair enough. I suppose it just hasn't taken me long to get to the point where I sympathize with the author... — chrylis -cautiouslyoptimistic-, Sep 25 '13 at 12:22

Jerry · Accepted Answer · 2013-09-26T05:15:58.350

You can use double negative lookaheads for this:

test(?!(?:(?!<_strong>).)*<_/strong>)

regex101 demo

This makes sure that the test is not followed by a <_/strong> (unless there's an opening <_strong> in between).

(?! ... ) is a negative lookahead. It prevents a match if the previous expression is followed by the expression inside the negative lookahead.

E.g.

a(?!b) will match all a not followed by b.

(?!(?:(?!<_strong>).)*<_/strong>) has two negative lookaheads. First we could say that there is (?!.*<_/strong>). When you have test(?!.*<_/strong>), this will match all test unless they have a <_/strong> after them. Now, this wouldn't work for the second sentence because even the first test has a <_/strong> after it.

The trick is that a test is considered to be within <_strong> tags only if there isn't an opening <_strong> tag between test and <_/strong>. That is where the .* turned into (?:(?!<_strong>).)*

You can play around in the regex101 demo site I linked in my answer earlier.

Thank you, that code works perfectly. However I'm below in knowledge that a noob. Can you explain me how it works exactly? — Esporas, Sep 26 '13 at 02:44
@user2814276 I added some more explanation to my answer, if it helps — Jerry, Sep 26 '13 at 05:16

Regular expression in java to match XML

1 Answers1