14 categories
See all →
  1. Home
  2. Developer Tools
  3. Regex Tester

Regex Tester

Test regular expressions with real-time match highlighting

/ /
g = global i = case-insensitive m = multiline s = dotAll
Advertisement

What is Regex Tester?

Free regex tester. Test regular expressions with live match highlighting, capture groups, and replace functionality. Developer utilities built for web developers, software engineers, and programmers. TekPulse's Regex Tester is completely free — no signup, no ads in results, and no data stored on our servers.

How to Use Regex Tester Online

  1. Enter your regular expression in the pattern field (without surrounding slashes)
  2. Select flags: g (global), i (case-insensitive), m (multiline), s (dotAll)
  3. Type or paste the test string in the large text area
  4. All matches are highlighted in real time with the match index, position, and captured groups listed below
  5. Use the replacement field to test regex substitutions and preview the replaced string

Regex Tester Formula & Calculation

Flags modify engine behavior: g (global), i (case-insensitive), m (multiline ^$), s (dotAll), u (unicode), y (sticky)

A regular expression is compiled once with a pattern and a set of flags that change how matching behaves — they don't change the pattern syntax itself. Without the 'g' flag, matching stops after the first hit; with it, matchAll() walks the whole string collecting every match and its capture groups.

Example: pattern (\d{3})-(\d{4}) against "Call 555-1234 or 555-5678" with flags 'g' finds two matches, each exposing capture group 1 (area code part) and group 2 (number part) separately — useful for extracting structured data out of free text.

Key Factors That Affect Your Result

  1. Global flag (g) — Required to find all matches instead of just the first. Also changes String.replace() to replace every occurrence instead of only the first.
  2. Case-insensitive (i) and multiline (m) — 'i' ignores letter case; 'm' makes ^ and $ match the start/end of each line (not just the whole string) — essential when testing multi-line log files or text blocks.
  3. Capture groups vs non-capturing — Parentheses (...) capture the matched substring for later reference ($1, $2, or match[1]); (?:...) groups a pattern without capturing, keeping output cleaner when you only need to group alternatives.
  4. Catastrophic backtracking — Nested quantifiers like (a+)+ can make matching take exponential time on certain inputs, freezing the browser tab. Test unusual/adversarial input if the regex will run on user-submitted data.

Why Use TekPulse Regex Tester?

Runs Entirely in Your Browser
Your pattern and text are tested with JavaScript's native RegExp engine — nothing leaves your device
Live Match Highlighting
Matches are highlighted in the text as you type, with a live valid/invalid regex status
Match List with Index
See every match's position and captured text in a numbered list
Find & Replace + Quick Patterns
Test replacements live, or start from ready-made patterns for email, URL, phone, date, IP and PIN code

Share & Embed

Help others discover this free tool. Share it or embed it on your website.

WhatsApp X/Twitter LinkedIn

Free to embed with attribution link to tekpulse.in

Related Guides

Regex Basics: A Practical Introduction

Frequently Asked Questions

A regular expression is a sequence of characters that defines a search pattern. Used for finding, matching, and replacing text. Example: \d{10} matches any 10-digit number (like a phone number).

. matches any character except newline. * means zero or more of the preceding element. Together .* matches any string of any length. Use .*? (lazy) to match as few characters as possible.

A basic email regex: ^[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}$. For production use, it is recommended to validate emails by sending a confirmation link rather than relying solely on regex.

g (global): find all matches, not just the first. i (case-insensitive): match regardless of case. m (multiline): ^ and $ match start/end of each line. s (dotAll): . matches newlines too.

Greedy (default): matches as much as possible. <.*> matches the longest possible string between < and >. Lazy (add ?): <.*?> matches the shortest possible. Lazy is needed when you want to match individual HTML tags.

Was this tool helpful?