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Regex to match a pattern, but exclude a set of words

Answer 1

+3 A:

If the regular expression implementation supports look-ahead or look-behind assertions, you could use the following:

Using a negative look-ahead assertion:
```
 \b(?!(?:cat|dog|sheep)\()\w+\(
```
Using a negative look-behind assertion:
```
 \b\w+\((?<!\b(?:cat|dog|sheep)\()
```

I added the \b anchor that marks a word boundary. So catdog( would be matched although it contains dog(.

But while look-ahead assertions are more widely supported by regex implementations, the regex with the look-behind assertion is more efficient since it’s only tested if the preceding regex (in our case \b\w+\() already did match. However the look-ahead assertion would be tested before the actual regex would match. So in our case the look-ahead assertion is tested whenever \b is matched.

Gumbo 2009-07-23 16:19:56

The second one is most likely efficient since it doesn't check every single position with a negative look-ahead (it's worth noting that they're negative.) Also, I'm thinking it might be better to put the negative look-behind after the parenthesis and include a parenthesis in the look-behind. This way, it will only perform an extra look-behind once it finds a possible match, rather than for every word in the string.

Blixt 2009-07-23 16:25:16

@Blixt: Good point.

Gumbo 2009-07-23 16:28:56

Also, your first regex will reject `catastrophe(`, `dogmatic(` and `sheepily(`. Your second one is saved from a similar error by the `\b` in the look-behind.

rampion 2009-07-23 16:33:24

Right, I had a go with > grep '\b(?!(?:cat|dog|sheep))\w+[(]' text.txt text.txtcat()dog()catdog()something()And it's not returning anything. I also had a go in textmate with the regular expression search but nada. I can see the logic behind the first statement though, perhaps this is a compatibility issue? I thought look-ahead was pretty standard. I've certainly been using it today in some form or another.

Huguenot 2009-07-23 16:37:19

ah, no carriage returns, the second text.txt indicates the file's contents.

Huguenot 2009-07-23 16:40:03

grep uses POSIX regular expressions, not PCRE, which are slightly different. I don't think the POSIX standard includes lookbehinds or lookaheads.

rampion 2009-07-23 16:40:15

Thank you, it was an unfortunate coincidence that both methods weren't working properly. Textmate had copied over a carriage return into the regular expression search box. Thanks very much for the help everyone

Huguenot 2009-07-23 16:45:46

I have modifed the first one to "\b(?!cat\(|dog\(|sheep\()\w+\(" to prevent the problem mentioned by Rampion and that seems to work. For some reason, it didn't like the word boundary in the second expression, so I changed it to a \W à la \b[A-Za-z]+\((?<!\W(?:cat\(|dog\(|rat\()) and that seems to have done the trick. Note that I had to change the third term to the same size as the others, fortunately this is not a problem.

Huguenot 2009-07-23 17:32:58

The first regex just needed grouping parens around the alternation: `\b(?!(?:cat|dog|sheep)\()\w+\(`

Alan Moore 2009-07-23 17:45:37

@Alan M: You’re right, thanks.

Gumbo 2009-07-23 18:32:22

Answer 2

A:

Do you really require this in a single regex? If not, then the simplest implementation is just two regexes - one to check you don't match one of your forbidden words, and one to match your \w+, chained with a logical AND.

ire_and_curses 2009-07-23 16:21:31

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Regex to match a pattern, but exclude a set of words

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