re-x 🔍⚡

AI-native regex CLI — Test, validate, explain. Built for coding agents.

re-x is a regex testing tool designed for AI coding agents. JSON output by default. Single binary. Zero dependencies.

AI agents generate regex patterns constantly — but they can't test them. re-x fixes that.

$ re-x test '(\d{3})-(\d{4})' 'Call 123-4567 now'
{
  "matched": true,
  "match_count": 1,
  "matches": [
    {
      "text": "123-4567",
      "start": 5,
      "end": 13,
      "captures": [
        {"group": 1, "text": "123", "start": 5, "end": 8},
        {"group": 2, "text": "4567", "start": 9, "end": 13}
      ]
    }
  ]
}

Why re-x?

Install

# Homebrew (macOS / Linux)
brew install re-x-ai/tap/re-x

# Cargo (Rust)
cargo install re-x

# Download binary from GitHub Releases
# https://github.com/re-x-ai/re-x/releases/latest

Commands

re-x test — Test a pattern

# Against a string
re-x test '\b\w+@\w+\.\w+\b' 'Email me at user@example.com'

# Against a file (streaming, handles large files)
re-x test 'ERROR|WARN' --file app.log --max-matches 20

# Via pipe
cat data.csv | re-x test '^\d{4}-\d{2}-\d{2},'

re-x replace — Preview replacements

re-x replace '(\w+)@(\w+)' '$1 [at] $2' 'user@example.com'
# → {"result": "user [at] example.com", "replacements_made": 1}

# Preview replacements on a file (never modifies the file)
re-x replace 'http://' 'https://' --file urls.txt

re-x validate — Check syntax & portability

re-x validate '(?<=\d{3})\w+'
# {
#   "valid": true,
#   "engine_required": "fancy-regex",
#   "portability": {
#     "rust_regex": false,
#     "pcre2": true,
#     "javascript": false,
#     "python_re": true,
#     "python_regex": true,
#     "go_regexp": false,
#     "java": true,
#     "dotnet": true,
#     "ruby": true
#   }
# }

re-x explain — Break down a pattern

re-x explain '^(?:https?://)?(?:www\.)?([^/]+)'
# Returns structured JSON with each token explained

re-x from-examples — Infer pattern from strings

re-x from-examples '2024-01-15' '2025-12-31' '2023-06-01'
# → [{"pattern": "\\d{4}-\\d{2}-\\d{2}", "confidence": 0.95}, ...]

re-x apply — Apply replacements to a file

# Dry-run first (shows what would change)
re-x apply 'http://' 'https://' --file urls.txt --dry-run

# Apply with backup (creates urls.txt.bak)
re-x apply 'http://' 'https://' --file urls.txt

# Apply without backup
re-x apply 'http://' 'https://' --file urls.txt --no-backup

# Multiline replacements (cross-line matching)
re-x apply '(?ms)^import.*?;$' 'use crate::*;' --file src/main.rs -m

re-x benchmark — Performance & ReDoS detection

re-x benchmark '(a+)+$' --input 'aaaaaaaaaaab'
# → {"catastrophic_backtracking": true, "warning": "..."}

AI Integration

Use with Claude Code (MCP)

# Register as MCP server
claude mcp add re-x -- re-x --mcp

Or add to .mcp.json:

{
  "mcpServers": {
    "re-x": {
      "type": "stdio",
      "command": "re-x",
      "args": ["--mcp"]
    }
  }
}

Use with Claude Code (bash — zero config)

Claude Code can call re-x directly via bash — just install it and it's available:

You: "Write a regex to extract emails from this log file and test it"
Claude Code: runs `re-x test '\b[\w.-]+@[\w.-]+\.\w+\b' --file app.log`

Skills

Copy skills/regex-testing/SKILL.md to your project's .claude/skills/ directory to teach Claude Code when and how to use re-x:

mkdir -p .claude/skills/regex-testing
cp skills/regex-testing/SKILL.md .claude/skills/regex-testing/

For Cursor, copy skills/cursorrules/.cursor/rules/regex-testing.mdc to your project's .cursor/rules/ directory.

Human-Friendly Mode

While re-x defaults to JSON (for AI), humans can use --format text:

re-x test '\d+' 'abc 123 def 456' --format text

Pattern: \d+
Engine:  regex (linear time)

Match 1: "123" [4..7]
Match 2: "456" [12..15]

2 matches found in 8μs

Design Decisions

JSON-first: Every command outputs structured JSON by default. AI agents parse JSON; humans can use --format text.

Dual engine: Simple patterns use the regex crate (linear time guaranteed). Patterns with lookahead/backreferences automatically use fancy-regex. You never need to think about it.

Safe by default: re-x replace previews changes but never modifies files. re-x apply writes to files but creates a .bak backup by default and supports --dry-run.

Zero dependencies: Single static binary. No Python, no Node.js, no runtime. Just download and run.

Performance

Built in Rust with the world-class regex crate:

Contributing

Contributions welcome!

git clone https://github.com/re-x-ai/re-x
cd re-x
cargo test
cargo run -- test '\d+' 'hello 123'

Sponsor

re-x is free and open source (MIT). Sponsoring keeps it maintained:

• 🚀 New features and language support
• 🐛 Quick bug fixes
• 📖 Better docs and AI Skills

💖 Sponsor on GitHub

License

MIT