Regex Tester Guide — How to Write and Test Regular Expressions
Updated October 2026 · 5 min read
Quick start: Open the free Regex Tester — type a pattern and test string to see matches highlighted in real time.
A regular expression (regex) is a sequence of characters defining a search pattern. They are used everywhere: form validation, log parsing, find-and-replace in code, URL routing and data extraction. The syntax looks intimidating but a small set of rules covers 90% of real-world use cases.
Core Regex Syntax
| Pattern | Meaning | Example |
|---|---|---|
| . | Any character except newline | c.t → cat, cut, c3t |
| * | 0 or more of preceding | ab* → a, ab, abb |
| + | 1 or more of preceding | ab+ → ab, abb (not a) |
| ? | 0 or 1 of preceding (optional) | colou?r → color, colour |
| ^ | Start of string | ^Hello → starts with Hello |
| $ | End of string | end$ → ends with "end" |
| [abc] | Character class | [aeiou] → any vowel |
| \d | Any digit (0–9) | \d{4} → 4-digit number |
| \w | Word character (a-z, A-Z, 0-9, _) | \w+ → any word |
| {n,m} | Between n and m repetitions | \d{3,5} → 3 to 5 digits |
Practical Patterns You Can Use Today
Email address:
^[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}$
US phone (flexible):
^\+?1?\s?\(?\d{3}\)?[\s.-]?\d{3}[\s.-]?\d{4}$
URL (http/https):
https?:\/\/[\w-]+(\.[\w-]+)+([\w.,@?^=%&:/~+#-]*[\w@?^=%&/~+#-])?
HEX colour code:
^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$
Flags — Global, Case-Insensitive, Multiline
Regex flags modify matching behaviour. The three most important: g (global — find all matches, not just first), i (case-insensitive), m (multiline — ^ and $ match line starts/ends, not just the whole string). Most regex testers let you toggle flags — test your pattern with and without g to see the difference.
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