What is an example of a matching and non-matching string?

A valid matching string is "This is the the best tool". A non-matching string is "This is the best tool".

Is this regex safe from ReDoS (Catastrophic Backtracking)?

Yes, this pattern is optimized for linear execution time and safe for browser-side validation without locking up the UI thread.

HomeRegex LibraryDuplicate Consecutive Word Regex

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Duplicate Consecutive Word Regex

Q: How do I test this regex in JavaScript and TypeScript?

Use regex.test(): const regex = /\b([a-zA-Z]+)\s+\1\b/i; const isValid = regex.test(inputString);

Finds accidental repeated words in editorial prose and code comments (e.g., "the the", "in in").

Regular Expression Pattern:

/\b([a-zA-Z]+)\s+\1\b/i

Valid Match Example:

This is the the best tool

Invalid / Non-Match Example:

This is the best tool

Regex Syntax Breakdown & Explanation

•\b([a-zA-Z]+) : Matches and captures an entire word at a word boundary
•\s+ : One or more whitespace spaces
•\1\b : Backreference matches the exact duplicate word

Implementation in Popular Languages

JavaScript / TypeScript

const regex = /\b([a-zA-Z]+)\s+\1\b/i;
const isValid = regex.test("This is the the best tool");
console.log(isValid); // true

Python (re)

import re

pattern = r"\b([a-zA-Z]+)\s+\1\b"
match = re.match(pattern, "This is the the best tool")
print(bool(match)) # True

Go (regexp)

package main
import (
  "fmt"
  "regexp"
)

func main() {
  re := regexp.MustCompile(`\b([a-zA-Z]+)\s+\1\b`)
  fmt.Println(re.MatchString("This is the the best tool"))
}

Frequently Asked Questions

Everything you need to know regarding specifications, syntax, and security best practices.

The regular expression pattern is /\b([a-zA-Z]+)\s+\1\b/i. Finds accidental repeated words in editorial prose and code comments (e.g., "the the", "in in").