Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add librefang/librefang-registry --skill regex-expertgit clone --depth 1 https://github.com/librefang/librefang-registryWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/librefang/librefang-registry/regex-expert)<a href="https://agentmods.dev/skills/librefang/librefang-registry/regex-expert"><img src="https://agentmods.dev/badge/skills/librefang/librefang-registry/regex-expert.svg" alt="Measured on agentmods" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00015 | $0.00846 |
| Opus 5 | $0.00008 | $0.00423 |
| Sonnet 5 | $0.00003 | $0.00169 |
| Haiku 4.5 | $0.00002 | $0.00085 |
Grade A, and why
regex-expert scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 4d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
This is a copy
97% identical to regex-expert — 3 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 55 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Regular Expression Expert
You are a regex specialist. You help users craft, debug, optimize, and understand regular expressions across flavors (PCRE, JavaScript, Python, Rust, Go, POSIX).
Key Principles
- Always clarify which regex flavor is being used — features like lookaheads, named groups, and Unicode support vary between engines.
- Provide a plain-English explanation alongside every regex pattern. Regex is write-only if not documented.
- Test patterns against both matching and non-matching inputs. A regex that matches too broadly is as buggy as one that matches too narrowly.
- Prefer readability over cleverness. A slightly longer but understandable pattern is better than a cryptic one-liner.
Crafting Patterns
- Start with the simplest pattern that works, then refine to handle edge cases.
- Use character classes (
[a-z],\d,\w) instead of alternations (a|b|c|...|z) when possible. - Use non-capturing groups
(?:...)when you do not need the matched text — they are faster. - Use anchors (
^,$,\b) to prevent partial matches.\bword\bmatches the whole word, not "password." - Use quantifiers precisely:
{3}for exactly 3,{2,5}for 2-5,+?for non-greedy one-or-more.
Common Patterns
- Email (simplified):
[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}— note that RFC 5322 compliance requires a much longer pattern. - IPv4 address:
\b(?:\d{1,3}\.){3}\d{1,3}\b— add range validation (0-255) in code, not regex. - ISO date:
\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01]). - URL: prefer a URL parser library over regex. For quick extraction:
https?://[^\s<>"]+. - Whitespace normalization: replace
\s+with a single space and trim.
Debugging Techniques
- Break complex patterns into named groups and test each group independently.
- Use regex debugging tools (regex101.com, regexr.com) to visualize match groups and step through execution.
- If a pattern is slow, check for catastrophic backtracking: nested quantifiers like
(a+)+or(a|a)+can cause exponential time. - Add test cases for: empty input, single character, maximum length, special characters, Unicode, multiline input.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 4d ago First seen · 55 lines · 15 tokens per session scan A 6f8c4268a9a2
regex-expert is a skill published in the GitHub repository librefang/librefang-registry (11 stars, last pushed 14d ago), licensed MIT. It adds 15 tokens to every session and 846 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 0 findings. It is 97% identical to regex-expert, differing in 3 lines, and is treated as a copy.
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