Regex to Match Trailing Whitespace

This regex detects trailing whitespace at the end of lines, a common code quality issue. Here is the token-by-token breakdown:

[^\S\n]+ — A negated character class with a double negation that effectively matches whitespace characters excluding newlines. Breaking it down: \S matches any non-whitespace character, and \n matches a newline. The negation [^...] inverts this, so the class matches characters that are NOT non-whitespace and NOT newlines. In other words, it matches whitespace characters that are not newlines: spaces, tabs, and other horizontal whitespace. The + quantifier requires one or more such characters.

$ — Anchors the match at the end of a line. With the m (multiline) flag, this matches at the end of each line rather than only at the end of the entire string.

The g flag enables global matching to find trailing whitespace on all lines, and the m flag enables multiline mode so $ matches at each line ending.

Trailing whitespace is considered undesirable in source code for several reasons: it creates unnecessary diffs in version control, it can cause issues in some programming languages and markup formats, it wastes bytes, and it is a common linting rule violation. Most code editors can be configured to automatically strip trailing whitespace on save.

This pattern is useful in code linters, pre-commit hooks, text editors, and CI/CD pipelines that enforce code style rules. It detects trailing spaces and tabs without matching the newline character itself, making it ideal for search-and-replace operations where you want to remove the whitespace but keep the line structure intact.