Question 1

What is a regular expression?

Accepted Answer

A regular expression (regex) is a sequence of characters that defines a search pattern. It is used for matching, finding, and replacing text in strings. Regex is supported in virtually every programming language.

Question 2

What are regex flags?

Accepted Answer

Flags modify how the regex engine matches. 'g' (global) finds all matches instead of stopping at the first. 'i' makes the match case-insensitive. 'm' (multiline) makes ^ and $ match line boundaries. 's' (dotAll) makes the dot character match newline characters.

Question 3

What are some common regex patterns?

Accepted Answer

Common patterns include email validation ([a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}), URL matching (https?://[^\s]+), phone numbers, IP addresses, and date formats. Use the preset buttons above to load these instantly.

Question 4

Is my data safe?

Accepted Answer

Yes. Everything runs entirely in your browser. No data is sent to any server. Your regex patterns and test strings never leave your device.

Question 5

What is the difference between PCRE and JavaScript regex?

Accepted Answer

PCRE (used in PHP, Python, etc.) supports features like lookbehind, possessive quantifiers, and named groups that JavaScript historically lacked. Modern JavaScript (ES2018+) now supports lookbehind and named capture groups, but some PCRE features like atomic groups and recursive patterns are still unavailable.

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