Regex-LE

Regex-LE: Zero Hassle Regex Extraction & Validation

Find, test, and validate the regex patterns in the current file

Literal patterns, RegExp constructors, ReDoS screening

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What it does

Open any file and run one of three commands. Extract lists every regex pattern found in the document. Test 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.

Configuring it by hand — any host with an MCP config file

Most hosts read a JSON config. Add one entry:

{

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

-y skips the install prompt on first run. Pin a version if you would rather not track releases — regex-le-mcp@2.6.0.

Prefer not to go through npx on every launch? Install it once and point at the binary instead:

npm install -g regex-le-mcp

{

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

It speaks MCP over stdio and needs no environment variables, no API key and no configuration of its own. To check it before wiring it into anything:

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

That prints the tool list and exits — if you see extract_patterns, the server works.

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 \w+)+@') is reported as written, and still flagged.
• Constructor calls whose pattern argument is a variable, or a template literal with a ${…} substitution. String literals, static template literals and String.raw`…` are read.
• 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: , 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).

Across a folder or a workspace

Extract and Validate read the document you have open. Each has a form that reads many files from disk and gives one report.

• The whole workspace: run Regex-LE: Extract Patterns from Workspace or Regex-LE: Validate Patterns in Workspace from the command palette.
• One folder: right-click it in the Explorer and choose Extract Patterns from Folder or Validate Patterns in Folder. The same two commands in the palette ask for a folder.

Extract

A project writes the same pattern in many places, so the report is the distinct patterns and where each one is, the most widely used first:

# Regex-LE workspace report

my-project · 3 file(s) read · 3 distinct pattern(s) in 3 file(s)

/\d+/g (2)

• src/a.ts · 1:11
• src/b.ts · 1:11

/\d+/ (1)

• src/c.py · 2:5

/^x$/ (1)

• src/a.ts · 2:11

Two patterns are the same when their text and flags are, so /\d+/g and /\d+/ are listed apart. A pattern is counted once per file, at the first place it is written there.

Validate

The report opens with a table of the files that hold something to look at, then lists the patterns that can hang:

# Regex-LE workspace report

my-project · 3 file(s) read · 5 pattern(s), 1 can hang, 1 not checked

1 other file(s) hold only patterns with no finding.

src/a.ts (1)

• 2:14 · /(a+)+$/ · can hang (high): exponential backtracking: 2000000 steps on 41 characters, against 229328 on 20 · complexity 0/100

Can hang means an input was found that drives the pattern into backtracking, the same search ReDoS screening describes. Not checked means the search could not answer for the pattern either way, because it uses a backreference or a lookaround. That is not a verdict on the pattern, and the report gives the reason.

A pattern written for another language's engine is read, not refused: a Python (?P \d{4}), an atomic group or a possessive quantifier is searched like any