Regex Flags Explainer

The g (global) flag makes the regex find all matches instead of stopping after the first one. Without g, methods like String.match() return only the first match. With g, they return an array of all matches. String.replace() with g replaces all occurrences; without it, only the first is replaced.

The i flag makes the regex match regardless of letter case. /hello/i matches 'Hello', 'HELLO', 'hElLo', and 'hello'. This is useful for user input validation where you don't want case to matter, like checking for keywords or usernames.

The m flag changes how ^ and $ anchors behave. Without m, ^ matches the start of the entire string and $ matches the end. With m, ^ matches the start of each line and $ matches the end of each line. This is essential when processing multi-line text where you want to match patterns at line boundaries.

The s flag (also called dotAll) makes the dot (.) match any character including newlines. Normally, . matches everything except \n (newline). With /s, . matches \n too, which is useful for matching patterns that span multiple lines like HTML tags or multi-line strings.

The u flag enables proper Unicode support. Without it, regex treats emoji and characters outside the Basic Multilingual Plane (BMP) as two surrogate pairs. With /u, \w, \b, and character classes work correctly with Unicode. Use /u whenever your input may contain non-ASCII characters or emoji.

The y flag makes the regex 'sticky' — it only matches at the exact position stored in regex.lastIndex. Unlike g which searches the whole string, y only succeeds if a match starts right at lastIndex. This is used in tokenizers and parsers where you need sequential matching at exact positions.