Java RegEx: Very Specific Match

Question

I'm trying to match a hyphen in all situations except when it is preceded by whitespace and proceeded by non-whitespace.

x-y   //match
x - y //match
x- y  //match
x -y  //not match

I want the regex to determine a match based on the above requirements but only capture the negative sign. Is this possible?

Thanks!

Edited for accuracy.

It seems you're example and requirement don't match. Could you be more accurate? — steinar
– steinar, Commented Mar 20, 2011 at 3:20

Cameron · Accepted Answer · 2011-03-20 03:48:28Z

4

It's possible using a negative lookbehind and a positive lookahead assertion (docs):

(?<!\s)-|-(?=\s)

Which says: Match any hyphen either (not preceded by whitespace) or (proceeded by whitespace). This is the same as saying "not (preceded by whitespace) and (proceeded by non-whitespace)", according to De Morgan's laws.

edited Mar 20, 2011 at 3:48

answered Mar 20, 2011 at 3:20

Cameron

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1 Comment

djs22 Over a year ago

Thanks Cameron. Sorry for the ambiguity in the question too; I changed it so my description matches my examples now.

ridgerunner · Accepted Answer · 2011-03-20 03:48:22Z

0

You could employ multiple alternatives, one for each case:

Pattern regex = Pattern.compile(
    "  (?<=\\S)-(?=\\S)  # x-y   //match\n" +
    "| (?<=\\s)-(?=\\s)  # x - y //match\n" +
    "| (?<=\\S)-(?=\\s)  # x- y  //match", 
    Pattern.COMMENTS);

This logic could probably be shortened (at the expense of some readability).

answered Mar 20, 2011 at 3:48

ridgerunner

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