Regular expression: match string containing only non-duplicate words

Question

I have this situation (Java code): 1) a line such as: "Wild adventure" should match. 2) a line with adjacent repeating words: "Wild wild adventure" should not match.

With this regex :. * \ b (\ w +) \ b \ s * \ 1 \ b. * I can match strings containing adjacent repeating words.

How to change the situation ie how to match strings that do not contain adjacent repeating words

Answer 1

Use a negative statement, (?!pattern) .

  String[] tests = { "A wild adventure", // true "A wild wild adventure" // false }; for (String test : tests) { System.out.println(test.matches("(?!.*\\b(\\w+)\\s\\1\\b).*")); } 

Explanation courtesy of Rick Measham explain.pl :

 REGEX: (?!.*\b(\w+)\s\1\b).* NODE EXPLANATION -------------------------------------------------------------------------------- (?! look ahead to see if there is not: -------------------------------------------------------------------------------- .* any character except \n (0 or more times (matching the most amount possible)) -------------------------------------------------------------------------------- \b the boundary between a word char (\w) and something that is not a word char -------------------------------------------------------------------------------- ( group and capture to \1: -------------------------------------------------------------------------------- \w+ word characters (az, AZ, 0-9, _) (1 or more times (matching the most amount possible)) -------------------------------------------------------------------------------- ) end of \1 -------------------------------------------------------------------------------- \s whitespace (\n, \r, \t, \f, and " ") -------------------------------------------------------------------------------- \1 what was matched by capture \1 -------------------------------------------------------------------------------- \b the boundary between a word char (\w) and something that is not a word char -------------------------------------------------------------------------------- ) end of look-ahead -------------------------------------------------------------------------------- .* any character except \n (0 or more times (matching the most amount possible)) 

see also

• regular-expressions.info/Lookarounds

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Note

Negative statements make sense only when there are other patterns that you want to positively match (see examples above). Otherwise, you can simply use the logical complement operator ! to negate matches with the same pattern you used before.

 String[] tests = { "A wild adventure", // true "A wild wild adventure" // false }; for (String test : tests) { System.out.println(!test.matches(".*\\b(\\w+)\\s\\1\\b.*")); }