[regex] Regular Expression For Duplicate Words

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?

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.

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

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

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.

Try this with below RE

• \b start of word word boundary
• \W+ any word character
• \1 same word matched already
• \b end of word

• ()* Repeating again

  public static void main(String[] args) {
  
      String regex = "\\b(\\w+)(\\b\\W+\\b\\1\\b)*";//  "/* Write a RegEx matching repeated words here. */";
      Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE/* Insert the correct Pattern flag here.*/);
  
      Scanner in = new Scanner(System.in);
  
      int numSentences = Integer.parseInt(in.nextLine());
  
      while (numSentences-- > 0) {
          String input = in.nextLine();
  
          Matcher m = p.matcher(input);
  
          // Check for subsequences of input that match the compiled pattern
          while (m.find()) {
              input = input.replaceAll(m.group(0),m.group(1));
          }
  
          // Prints the modified sentence.
          System.out.println(input);
      }
  
      in.close();
  }
  

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

The below expression should work correctly to find any number of consecutive words. The matching can be case insensitive.

String regex = "\\b(\\w+)(\\s+\\1\\b)*";
Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE);

Matcher m = p.matcher(input);

// Check for subsequences of input that match the compiled pattern
while (m.find()) {
     input = input.replaceAll(m.group(0), m.group(1));
}

Sample Input : Goodbye goodbye GooDbYe

Sample Output : Goodbye

Explanation:

The regex expression:

\b : Start of a word boundary

\w+ : Any number of word characters

(\s+\1\b)* : Any number of space followed by word which matches the previous word and ends the word boundary. Whole thing wrapped in * helps to find more than one repetitions.

Grouping :

m.group(0) : Shall contain the matched group in above case Goodbye goodbye GooDbYe

m.group(1) : Shall contain the first word of the matched pattern in above case Goodbye

Replace method shall replace all consecutive matched words with the first instance of the word.

This expression (inspired from Mike, above) seems to catch all duplicates, triplicates, etc, including the ones at the end of the string, which most of the others don't:

/(^|\s+)(\S+)(($|\s+)\2)+/g, "$1$2")

I know the question asked to match duplicates only, but a triplicate is just 2 duplicates next to each other :)

First, I put (^|\s+) to make sure it starts with a full word, otherwise "child's steak" would go to "child'steak" (the "s"'s would match). Then, it matches all full words ((\b\S+\b)), followed by an end of string ($) or a number of spaces (\s+), the whole repeated more than once.

I tried it like this and it worked well:

var s = "here here here     here is ahi-ahi ahi-ahi ahi-ahi joe's joe's joe's joe's joe's the result result     result";
print( s.replace( /(\b\S+\b)(($|\s+)\1)+/g, "$1"))         
--> here is ahi-ahi joe's the result

This is the regex I use to remove duplicate phrases in my twitch bot:

(\S+\s*)\1{2,}

(\S+\s*) looks for any string of characters that isn't whitespace, followed whitespace.

\1{2,} then looks for more than 2 instances of that phrase in the string to match. If there are 3 phrases that are identical, it matches.

The widely-used PCRE library can handle such situations (you won't achieve the the same with POSIX-compliant regex engines, though):

(\b\w+\b)\W+\1

Here is one that catches multiple words multiple times:

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

Regex to Strip 2+ duplicate words (consecutive/non-consecutive words)

Try this regex that can catch 2 or more duplicates words and only leave behind one single word. And the duplicate words need not even be consecutive.

/\b(\w+)\b(?=.*?\b\1\b)/ig

Here, \b is used for Word Boundary, ?= is used for positive lookahead, and \1 is used for back-referencing.

Example Source

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.