Regex only match if character is not included directly before desired string

Question

I'm trying to solve this CodingBat problem:

Return true if the given string contains an appearance of "xyz" where the xyz is not directly preceeded by a period (.). So "xxyz" counts but "x.xyz" does not.

xyzThere("abcxyz") → true
xyzThere("abc.xyz") → false
xyzThere("xyz.abc") → true

I'm trying to solve this with a regex, but I'm unsure how to handle where the xyz is not directly preceeded by a period requirement.

My solution for the problem without the constraint is this:

public boolean xyzThere(String str) {
    return str.matches(".*xyz.*");
}

Any idea how to handle the said constraint with a regex?

Negative lookbehind is what you are looking for. But to use it you also need to know about it limits — Pshemo
– Pshemo, Commented Nov 18, 2012 at 16:26

Kelvin · Accepted Answer · 2012-11-18 16:32:15Z

13

A negated character class should do the trick: str.matches(".*(?:^|[^.])xyz.*")

Here we're using a non capturing group (?:^|[^.]) to ensure that we match either at the start of the string ^, or at any position that isn't a period [^.]

edited Nov 18, 2012 at 16:32

answered Nov 18, 2012 at 16:23

Kelvin

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Comments

nhahtdh · Accepted Answer · 2012-11-18 16:32:41Z

8

I personally used this solution, but there are quite a number of other variants:

str.matches("(.*[^.])?xyz.*")

I just make sure that if there is anything in front of xyz, then the period . does not immediately precede.

You can also write a look-behind solution:

str.matches(".*(?<!\\.)xyz.*");

(?<! ) part is negative lookbehind, and \\. (literal period) is the pattern that we want to check against.

answered Nov 18, 2012 at 16:32

nhahtdh

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7 Comments

nhahtdh Over a year ago

@KelvinMackay: Java matches assume anchor.

nhahtdh Over a year ago

I pass the test cases on CodingBat, though. Please provide test case that fails the code.

nhahtdh Over a year ago

@KelvinMackay: The thing is the regex I wrote are equivalent to the version that has ^ in front and $ at the back, courtesy of matches in String class. Your argument would be correct if the anchors were not assumed.

Kelvin Over a year ago

I understand that, but that's not why it doesn't work when I try it. Anyway, it's not important - it works on CodingBat so your answer's correct :)

Woot4Moo Over a year ago

I do believe that forces a compile every time using str.matches(arbitraryRegEx);

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