fcp-regex

fcp-regex

MCP server for regex construction via named fragment composition.

What It Does

LLMs understand regex semantics perfectly — they know what they want to match. But they consistently fail at regex syntax: unbalanced groups, unescaped metacharacters, broken alternations. fcp-regex solves this by letting LLMs build regexes through named fragments with a flat element syntax. No raw regex, no paren balancing, no escaping burden. Built on the FCP framework.

Written in Rust using rmcp for MCP transport and regex-syntax for compilation.

Quick Example

regex_session('new "Semver Parser" flavor:pcre')

regex([
  "define digits any:digit+",
  "define version digits lit:. digits lit:. digits",
  "define prerelease lit:- chars:a-zA-Z0-9-.+",
  "define semver version opt:prerelease",
  "compile semver anchored:true"
])

Produces: ^\d+\.\d+\.\d+(?:-[a-zA-Z0-9\-.]+)?$

regex_query('test semver against:1.2.3-alpha.1')
→ MATCH

regex_query('test semver against:v1.2')
→ NO MATCH

Available MCP Tools

Tool	Purpose
regex(ops)	Batch mutations — define fragments, import patterns, compile to regex
regex_query(q)	Inspect — show, test, explain, list fragments and library
regex_session(action)	Lifecycle — new, close, status, undo, redo
regex_help()	Full reference card

Verb Reference — Mutations

Verb	Syntax
define	define NAME ELEMENT [ELEMENT...]
from	from SOURCE [as:ALIAS]
compile	compile NAME [flavor:F] [anchored:bool]
drop	drop NAME
rename	rename OLD NEW

Verb Reference — Queries

Verb	Syntax
show	show NAME — fragment tree + compiled regex
test	test NAME against:STRING — test match
list	list — all fragments
list library	list library [category:NAME] — pattern library
get	get PATTERN — library pattern detail
map	map — fragment + library overview
stats	stats — session statistics

Verb Reference — Session

Verb	Syntax
new	new "Title" [flavor:pcre]
close	close
status	status
undo	undo [to:CHECKPOINT]
redo	redo
checkpoint	checkpoint NAME

Element Syntax

Element	Meaning	Example
<name>	Reference another fragment	digits
lit:<chars>	Literal (auto-escaped)	lit:. lit:@
any:<class><quant>	Character class	any:digit+
none:<class><quant>	Negated class	none:whitespace+
chars:<set><quant>	Custom character set	chars:a-z0-9+
not:<set><quant>	Negated custom set	not:@/+
opt:<name>	Optional	opt:prerelease
alt:<a>|<b>	Alternation	alt:ipv4|ipv6
cap:<name>	Capture group	cap:domain
cap:<label>/<name>	Named capture	cap:port/digits
sep:<name>/<lit>	Separated repeat	sep:octet/lit:.
raw:<regex>	Raw regex passthrough	raw:(?<=\b)

Classes: digit alpha alphanumeric word whitespace any

Quantifiers: + (1+) * (0+) ? (0..1) {N} {N,M} {N,}

Pattern Library

56 curated patterns across 8 categories: uri, email, datetime, identifiers, network, http, common, data.

regex(["from semver"])
regex_query('list library')
regex_query('get rfc3986:uri')

Installation

Build from source

cargo install --git https://github.com/os-tack/fcp-regex

MCP Client Configuration

{
  "mcpServers": {
    "fcp-regex": {
      "command": "fcp-regex"
    }
  }
}

Architecture