Regex Tester: How to Write and Test Regular Expressions Online

Regular expressions look like someone fell asleep on a keyboard. A string like ^\w+@[\w.-]+\.\w{2,}$ makes perfect sense once you know how to read it, but at first glance it's just noise. That's why a regex tester exists. You type a pattern, paste some text, and immediately see what matches.

This guide covers everything you need to go from confused to confident with regex. You'll learn the syntax, the most common patterns, and how to test regex quickly using Morphkit's Regex Tester.

What Are Regular Expressions?

A regular expression (regex) is a sequence of characters that defines a search pattern. Think of it as a super-powered find function. Where a normal search finds exact text, regex lets you search for patterns: any email address, any phone number, any date in a specific format.

Regex is built into almost every programming language: JavaScript, Python, Java, PHP, Ruby, Go. It's also baked into command-line tools like grep and sed, database queries, text editors, and form validation logic.

Regex Syntax Crash Course

Character Classes

\d matches any digit (0-9)
\w matches any word character (letters, digits, underscore)
\s matches any whitespace
. matches any character except a newline
[abc] matches a, b, or c
[^abc] matches anything that isn't a, b, or c

Quantifiers

+ means one or more
* means zero or more
? means zero or one (optional)
{3} means exactly 3
{2,5} means between 2 and 5

Anchors

^ matches the start of a line
$ matches the end of a line
\b matches a word boundary

Groups and Alternation

(abc) creates a capturing group
| means "or"
(?:abc) is a non-capturing group

Regex Flags

g (global) finds all matches, not just the first
i (case-insensitive) treats uppercase and lowercase as the same
m (multiline) makes ^ and $ match line starts/ends
s (dotAll) makes . match newlines too

How to Test Regex with Morphkit

Open the Regex Tester
Type your pattern in the regex input field
Paste your test text below
Watch matches highlight in real time

Common Regex Patterns

Email: ^[\w.-]+@[\w.-]+\.\w{2,}$ Phone (US): ^$?\d{3}$?[-.\s]?\d{3}[-.\s]?\d{4}$ URL: https?://[\w.-]+(?:/[\w./?&=%#-]*)? IP Address: \b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b Date (YYYY-MM-DD): \d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])

Common Regex Mistakes

Greedy vs. Lazy Matching. By default, quantifiers are greedy. Add ? to make them lazy.

Forgetting to Escape Special Characters. The pattern file.txt also matches "fileAtxt" because . means any character. Write file\.txt instead.

Not Anchoring Your Pattern. Without ^ and $, your pattern matches substrings inside larger text.

Overcomplicating the Pattern. If your pattern takes more than a minute to explain, break it into smaller steps.

Start Testing Your Patterns

Open Morphkit's Regex Tester, paste in some sample text, and start building patterns. It works entirely in your browser. Nothing gets uploaded.

If your workflow involves replacing matched text, the Find and Replace tool supports regex-powered substitutions. And the Text Diff tool shows every change side by side.