Regex Tester — Build and Test Regular Expressions Quickly

Regular expressions are one of the most powerful text-processing tools — and one of the most intimidating to write. The Regex Tester on THRJ gives you a live environment to write, test, and debug patterns with real-time match highlighting and replacement previews. Open the Regex Tester.

What Is a Regular Expression?

A regular expression (regex) is a sequence of characters that defines a search pattern. It can match email addresses, phone numbers in many formats, line anchors, repeated patterns, optional groups, and much more. Regex is supported in JavaScript, Python, Java, Go, Ruby, and many editors and CLI tools.

Overview

Regex Tester provides a live environment to write regular expressions, test them against sample text, and visualize matches. It shows capture groups, replacement previews, and supports common flags like g, i, m, s, and u.

Quick Steps

1. Open the Regex Tester page: /regex-tester.
2. Enter or paste sample text into the input area.
3. Type your regular expression into the pattern field.
4. Toggle flags like i (ignore case) or g (global) as needed.
5. See matches highlighted live and use replacement preview to test substitutions.

Features

• Live highlighting of matches and capture groups
• Replacement preview to test substitutions safely
• Flag toggles and common pattern snippets
• Export your pattern and sample text for sharing

Understanding Regex Flags

Flag	Symbol	Effect
Global	g	Find all matches, not just the first
Ignore case	i	Match regardless of case
Multiline	m	^ and $ match line starts/ends
Dot all	s	. matches newline characters too
Unicode	u	Enable full Unicode matching

Common Regex Patterns Reference

Pattern	Purpose	Example
^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$	Email address	user@example.com
https?:\/\/[^^\s]+	URL (http or https)	https://example.com/path
\+?[\d\s\-().]{7,15}	Phone number (flexible)	+1 (555) 123-4567
\b\d{4}-\d{2}-\d{2}\b	ISO date (YYYY-MM-DD)	2024-07-15
^\s*$	Blank line	Lines with only spaces

How to Use Replacement Mode

Replacement preview lets you test String.replace() or sed-style substitutions before applying them.

Example — Normalize dates:

Pattern: (\d{4})-(\d{2})-(\d{2})
Replacement: $2/$3/$1
Result: 2024-07-15 → 07/15/2024

Step-by-Step Scenarios

Validating a form field pattern

1. Paste sample values into the input area.
2. Write your validation regex (e.g., ZIP code).
3. Toggle the g flag to see all matches.
4. Adjust until only valid entries are highlighted.

Cleaning up copied data

1. Paste messy text from a spreadsheet.
2. Write a pattern to match unwanted prefixes.
3. Use replacement mode with an empty replacement to strip them out.
4. Copy the cleaned result.

Tips & Common Mistakes

• Start simple and add complexity progressively.
• Escape literal special characters like ., *, +, ?, (, ).
• If you see no matches, check your flags — especially g.
• Use named capture groups ((?<name>...)) for clarity in complex replacements.

Privacy

All processing is local to your browser. Your patterns and test text never leave your device. Avoid pasting sensitive data if you are concerned about clipboard history.

Frequently Asked Questions

Do I need programming knowledge?
No. The tester is usable by developers and non-developers alike. The quick-reference above covers common use cases.

What's the difference between a match and a capture group?
A match is the full text the pattern matched; capture groups (parentheses) extract sub-parts of that match.

---

Published by THRJ Tech