Question 1

What regex flavour does the Regex Tester support?

Accepted Answer

ECMAScript / JavaScript regex (the same flavour used by Node.js, Chrome, Firefox, Safari, and Edge). This is similar but not identical to Python's re or PCRE. Most syntax is portable; differences include lookbehind support and Unicode property escapes. For PCRE-specific features, use a dedicated PCRE tool.

Question 2

Why is my pattern returning no matches?

Accepted Answer

Common causes: missing g flag (without it only the first match is returned, but the tester handles both modes), forgotten escaping of special characters (. + * ? ( ) [ ] { } | \ ^ $ all need escaping if you want the literal character), greedy quantifier eating too much (try the lazy variant by adding ? after + or *), or anchors (^ and $) preventing match if the pattern is meant to find within a larger string.

Question 3

Are my pattern and test data sent anywhere?

Accepted Answer

No. The Regex Tester runs entirely client-side in your browser. Pattern, flags, and test string are never transmitted to or stored on any server. Refresh the page and the inputs are gone. Safe to use with production log data, customer email lists, internal IDs, or any other sensitive payload.

Question 4

Does it support named capture groups?

Accepted Answer

Yes. Named groups using (?<name>...) syntax are surfaced separately in the match details list. Each match shows the group name and its captured value. Named groups are supported in modern browsers (ES2018+). For older runtime support, use numbered capture groups instead.

Question 5

How do I match a Pakistani phone number with this tool?

Accepted Answer

A practical pattern: ^(?:\+92|0)3\d{2}[\s-]?\d{7}$ for Pakistani mobile numbers (matches +92 3XX XXXXXXX or 03XX XXXXXXX with optional space or dash). Toggle the i flag if you want case-insensitive (irrelevant for digits but useful for mixed-case input). See our Regex Cheatsheet blog post for more ready-made patterns including CNIC, email, URL, and more.

Question 6

Why does my regex behave differently in the browser vs Node.js vs Python?

Accepted Answer

Different runtimes implement slightly different regex grammars. JavaScript / Node.js / browsers use ECMAScript regex (what this tester implements). Python's re module is similar but has minor differences (e.g., named groups historically used (?P<name>...)). PCRE (PHP, many CLI tools) is much richer and supports recursion. POSIX (basic grep, awk by default) is minimal. When porting a regex between languages, always re-test in the target environment.

Question 7

What is the difference between greedy and lazy quantifiers?

Accepted Answer

Greedy quantifiers (* and + by default) match as much as possible. Lazy quantifiers (*? and +?) match as little as possible. Example: on input

first

second

, greedy

.+

matches the whole string while lazy

.+?

matches just

first

. For extracting tags, attributes, or quoted strings, lazy is usually what you want.

Question 8

Why is the Regex Tester limited to 100 matches in display?

Accepted Answer

For very large inputs with thousands of matches, rendering every match would slow the browser. The tester computes all matches but displays the first 100 in the structured list. The highlighted-matches view in the test string display still highlights every match. If you genuinely need all matches programmatically, copy the pattern and run it in your code.

Regex Tester (Pakistani Phone, CNIC, Email, URL Patterns)

How to use the Regex Tester (Pakistani Phone, CNIC, Email, URL Patterns)

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