Regex-LE: Zero Hassle Regex Extraction & Validation

Find, test, and validate the regex patterns in the current file
Literal patterns, RegExp constructors, ReDoS screening

Useful? A star or rating is how other developers find it — ★ GitHub · ★ Open VSX · ★ Marketplace

What it does

Open any file and run one of three commands. Extract lists every regex pattern found in the document. Test (Ctrl+Alt+R / Cmd+Alt+R) runs a found — or manually entered — pattern against the file content and reports matches with real line/column positions and capture groups (named groups included). Validate checks every found pattern for syntax errors and screens it for catastrophic backtracking, reporting the input that causes it. Works in VS Code and VS Code–based editors like Cursor and VSCodium (installable from Open VSX).

Install

Use it from an AI agent

The same engine runs as an MCP server, so an agent can call it directly instead of you running a command.

extract_patterns(content, format?, filename?, maxResults?)

Returns every pattern with its flags, 1-based position and a ReDoS verdict, so "are any of the regexes in this file dangerous?" is one call rather than two. A verdict that reports a blow-up carries the witness that caused it, so an agent can check the finding instead of trusting it.

The server takes content and returns data — it reads no files and makes no network requests of its own. Published as regex-le-mcp on npm and as io.github.nolindnaidoo/regex-le in the MCP registry.

{
  "mcpServers": {
    "regex-le": {
      "command": "npx",
      "args": ["-y", "regex-le-mcp"]
    }
  }
}

npm install -g regex-le-mcp

{
  "mcpServers": {
    "regex-le": { "command": "regex-le-mcp" }
  }
}

echo '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | npx -y regex-le-mcp

What gets extracted

Extraction scans the whole document, so constructors split across lines are found too. The document's language chooses which spellings to look for:

A language nothing recognises is not a refusal — every spelling above is looked for. Naming it buys precision: a Python file is not scanned for bare /…/, so #!/usr/bin/env python stops reading as a pattern.

What is deliberately not extracted:

• Division, dates, and filesystem paths (a / b, 10/29/2025, /usr/local/bin): a / preceded by an identifier, number, ), ], ., or another / is not treated as a regex — after keywords like return, it is. That question is only asked where a bare /…/ is legal.
• Candidates that are not a well-formed regular expression in any of these languages, or with invalid/duplicate flags. Another language's spelling is not a syntax error: re.compile(r'(?P<word>\w+)+@') is reported as written, and still flagged.
• Constructor calls whose pattern argument is a variable or template literal (only literal string arguments are visible to a text scanner).
• Flags, on anything but a JavaScript literal or constructor: every other language sets them with constants, builder methods or an inline (?i) rather than a string argument.
• Anything written in a comment or a string. A JSDoc block explaining a hazard, a commented-out line, a Python docstring with an example — none of them is code, and reporting one fails a build over a sentence. The rule is about where a candidate starts, so re.compile(r"(a+)+b") keeps its quoted argument while a docstring holding that whole line is prose. Only when the language is known: a document nothing recognises is scanned as written, because a comment rule guessed from the wrong grammar would drop real patterns instead of phantom ones.

Duplicate pattern+flags pairs are listed once. This is lexing by heuristic, not a parser for nine languages: a slash inside a string can still be picked up when its context looks expression-like.

ReDoS screening

Validate (and Test, before running a risky pattern) reports a pattern only when an input was found that demonstrably drives it into catastrophic backtracking — and reports that input alongside it, as the witness.

Your pattern is never run. It is compiled to an automaton, and that automaton is walked the way a backtracking engine walks one — depth-first, every edge in order, a dead end unwound rather than remembered — while the steps are counted. An attack string is built, pumped at two lengths, and measured against a step budget. So a finding is falsifiable: run the witness and watch.

Nothing is reported on the strength of how a pattern is shaped. Shape is a poor predictor in both directions: ^[a-z0-9]+(?:-[a-z0-9]+)*$ looks dangerous and is not, because every iteration must eat a - the inner class cannot produce, while (.*a){20} looks bounded and is not. A separator forcing the split is a fact about strings, so no test on syntax settles it.

Silence is not a clearance. A pattern this cannot read — a backreference, lookaround, syntax it does not parse — comes back as not decided: <reason>, never as safe.

The reports also include a rough performance score based on execution time relative to input size — treat it as a hint, not a benchmark (memory is not measured).

The CLI

The same lint runs from a terminal or a shell pipeline: a Rust CLI in crate/, sharing one corpus with the extension — crate/fixtures/ — so the two can never read a document differently.

regex-le .                      # every vulnerable pattern in the tree
regex-le --severity high src/   # only the exponential shapes
regex-le --all src/             # every pattern, vulnerable or not
regex-le mcp                    # the same lint over MCP on stdio