Markdown Link Regex Tester

The Pattern

``regex \[([^\]]+)\]\(([^)\s]+)(?:\s+"([^"]*)")?\) `

Flags: g

What It Accepts and Rejects

Three Capture Groups

Group 1 is the link text, group 2 the URL, group 3 the optional title. The text class [^\]]+ matches anything that is not a closing bracket, and the URL class [^)\s]+ stops at a space or the closing parenthesis.

Note that the image syntax  also matches — the leading ! is simply not part of the pattern. Distinguishing links from images needs (? before the bracket, where lookbehind is supported.

The Limits of This Pattern

Markdown is not a regular language, and links are where that bites hardest. A URL containing a parenthesis — very common in Wikipedia links — terminates the match early. Nested brackets in link text break group 1. Reference-style links ([text][ref]) use a different syntax entirely and never match.

For extracting links from a handful of files this is fine. For anything that must be correct, parse with a CommonMark implementation and walk the AST.

Testing Before Shipping

A regex that has only been tried against inputs you expect to match is untested. Every pattern needs three kinds of case:

1. Valid inputs that should match, including the awkward-but-legal ones. 2. Invalid inputs that should not, especially near-misses that differ by one character. 3. Adversarial inputs — very long strings, unusual Unicode, and nesting that could trigger catastrophic backtracking.

Paste your own examples into the tester above and watch which lines highlight. A pattern that matches everything you throw at it is usually too permissive rather than correct.

Catastrophic Backtracking

Nested quantifiers over overlapping character classes — (a+)+, (\w+\s?)* — can take exponential time on a non-matching input. On a server that is a denial-of-service bug, not a performance issue. If a pattern is applied to user input, bound the input length first and prefer explicit alternation over nested repetition.

Anchors, Greediness and Backtracking

Three behaviours account for most regex surprises, and this pattern shows all three.

Anchors. ^ and $ pin the match to the start and end of the input. Without them, \d{3} matches the 123 inside abc123def. With m in the flags — as here — they pin to each *line* instead, which is what lets one pattern be tested against a list.

Greediness. .* takes as much as it can and gives back only when forced; .*? takes as little as possible. On , the pattern <.*> matches the whole string and <.*?> matches just .

Backtracking. When a match fails, the engine reverses and tries other splits. Nested quantifiers like (a+)+ make that exponential, and a 30-character input can hang a server — a class of denial of service known as ReDoS. Avoid nesting quantifiers, and prefer explicit character classes over . wherever you can.

Testing It Properly

`javascript const pattern = /\[([^\]]+)\]\(([^)\s]+)(?:\s+"([^"]*)")?\)/g;

// A global regex keeps lastIndex between calls, so reusing one across // test() calls returns alternating true/false on the same input. pattern.lastIndex = 0;

// Named groups make the result readable const named = /(?\d{4})-(?\d{2})/; const { groups } = '2026-08'.match(named); `

Write the failing cases first. A pattern that accepts everything valid is easy; one that also rejects everything invalid is the hard half, and it is where the bugs are.

When Not to Use a Regex

Structured formats have parsers, and the parser is always more correct: new URL() for URLs, DOMParser for HTML, JSON.parse` for JSON, a date library for dates. Reach for a regex to *find* things in unstructured text, not to validate something a parser understands.