Regular Expression For Consecutive Duplicate Words

Question

I'm a regular expression newbie, and I can't quite figure out how to write a single regular expression that would "match" any duplicate consecutive words such as:

Paris in the the spring.

Not that that is related.

Why are you laughing? Are my my regular expressions THAT bad??

Is there a single regular expression that will match ALL of the bold strings above?

Answer 1

Since some developers are coming to this page in search of a solution which not only eliminates duplicate consecutive non-whitespace substrings, but triplicates and beyond, I'll show the adapted pattern.

Pattern: /(\b\S+)(?:\s+\1\b)+/ (Pattern Demo)
Replace: $1 (replaces the fullstring match with capture group #1)

This pattern greedily matches a "whole" non-whitespace substring, then requires one or more copies of the matched substring which may be delimited by one or more whitespace characters (space, tab, newline, etc).

Specifically:

• \b (word boundary) characters are vital to ensure partial words are not matched.
• The second parenthetical is a non-capturing group, because this variable width substring does not need to be captured -- only matched/absorbed.
• the + (one or more quantifier) on the non-capturing group is more appropriate than * because * will "bother" the regex engine to capture and replace singleton occurrences -- this is wasteful pattern design.

*note if you are dealing with sentences or input strings with punctuation, then the pattern will need to be further refined.

Answer 2

The example in Javascript: The Good Parts can be adapted to do this:

var doubled_words = /([A-Za-z\u00C0-\u1FFF\u2800-\uFFFD]+)\s+\1(?:\s|$)/gi;

\b uses \w for word boundaries, where \w is equivalent to [0-9A-Z_a-z]. If you don't mind that limitation, the accepted answer is fine.

Answer 3

Here is one that catches multiple words multiple times:

(\b\w+\b)(\s+\1)+

Answer 4

I believe this regex handles more situations:

/(\b\S+\b)\s+\b\1\b/

A good selection of test strings can be found here: http://callumacrae.github.com/regex-tuesday/challenge1.html

Answer 5

Use this in case you want case-insensitive checking for duplicate words.

(?i)\\b(\\w+)\\s+\\1\\b

Answer 6

No. That is an irregular grammar. There may be engine-/language-specific regular expressions that you can use, but there is no universal regular expression that can do that.