Question

I have this situation(Java code):
1) a string such as : “A wild adventure” should match.
2) a string with adjacent repeated words: “A wild wild adventure” shouldn’t match.

With this regular expression: .* \b(\w+)\b\s*\1\b.* i can match strings containing adjacent repeated words.

How to reverse the situation i.e how to match strings which do not contain adjacent repeat words

Answer 1

Use negative lookahead assertion, (?!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’s 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 (a-z, A-Z, 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

Related questions

• using regular expression in Java
  • Uses negative lookahead to ensure a string doesn’t have a character occuring more than once
• Java split is eating my characters.
  • Many examples of using assertions
• How do I convert CamelCase into human-readable names in Java?
  • Very instructive example of using lookarounds

---

Note

Negative assertions only make sense when there are also other patterns that you want to positively match (see examples above). Otherwise, you can just use boolean complement operator ! to negate matches with whatever pattern you were using 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.*"));
}