How to use the Regex Expert agent skill

What the Regex Expert skill makes the agent do

Once the skill is active, the agent treats regular expressions as a specialist task. It first asks which regex flavor you use, because features such as lookaheads, named groups, and Unicode support differ between engines. It starts with the simplest pattern that works, then refines it for edge cases.

Every pattern comes with a plain-English explanation, and the agent suggests test inputs that should match and inputs that should not. It also looks for slow patterns that suffer from catastrophic backtracking and warns against using regex to parse HTML, XML, or JSON.

• Asks which flavor you are using before writing a pattern
• Explains each pattern in plain English
• Suggests matching and non-matching test inputs, including empty input, Unicode, and multiline text
• Finds and fixes patterns that can backtrack catastrophically
• Points out misuse, such as parsing HTML or JSON with regex

Install the Regex Expert skill

Download the regex-expert ZIP from this page and unzip it. The skill is a folder containing a SKILL.md file. You can install it in one of two ways.

For a personal setup, place the unzipped folder in ~/.claude/skills/. To share it with one project, place it in that project's .claude/skills/ folder. Alternatively, in the Claude apps, open Settings, go to Capabilities, and upload the skill under Skills.

• Personal: ~/.claude/skills/
• Single project: the project's .claude/skills/ folder
• Claude apps: Settings → Capabilities → Skills, then upload

Trigger the skill with natural requests

You do not need a special command. Describe what you need in ordinary language and mention regex or the pattern you are working with. Name your flavor up front to skip the clarifying question, and paste sample text that should and should not match.

Good requests say what the text looks like, what you want to capture or reject, and which language you are using.

• "Write a regex for ISO dates in Python. It should match 2024-03-15 but not 2024-13-01."
• "Explain this JavaScript pattern step by step: ^\w+@\w+\.com$"
• "My regex matches too much in these log lines. Help me narrow it down."
• "This pattern runs very slowly on long input. Check it for catastrophic backtracking."

Examples of what you get back

For a date check, the skill's guidance points to a pattern such as \d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01]). You should expect a plain-English breakdown of each part, examples that match, and examples that fail. For a slow pattern, expect the agent to look for nested quantifiers such as (a+)+ and suggest a safer structure.

The skill also recommends using character classes like [a-z], \d, and \w instead of long alternations, and non-capturing groups (?:...) when you do not need the captured text.

What the skill needs and how it runs

The skill lists no requirements. It does not run scripts and does not use the network. It consists of instructions in SKILL.md, so the agent works from that guidance and from the text you give it.

The skill points you to external regex testers, regex101.com and regexr.com, for visualizing matches and stepping through execution. Those sites are not part of the skill, so use them at your own discretion.

• Requires: nothing listed
• Runs scripts: no
• Uses network: no

Safety notes and when to use a different tool

Because the skill runs no scripts and uses no network access, its main risk is wrong advice rather than system changes. Test every pattern against your real data before relying on it.

The skill itself says regex alone is not enough for complex formats. For production validation of email addresses, URLs, or phone numbers, use a dedicated validation library and use regex only as a quick pre-filter. For HTML, XML, or JSON, use a real parser.

• Do not validate production email, URL, or phone input with regex alone
• Use a real parser for HTML, XML, or JSON
• Escape special characters in user input before embedding it in a pattern
• Remember that . does not match newlines by default in most flavors

Tips for better results

Give the agent a sample of real input, including the lines that should be rejected. A pattern that matches too broadly is as buggy as one that matches too narrowly, so both kinds of example help.

Ask for readability. The skill prefers a slightly longer pattern you can understand over a cryptic one-liner, and it recommends compiling patterns once and reusing them rather than recompiling inside loops.

• Always state the regex flavor
• Provide both matching and non-matching samples
• Ask for named groups to test parts separately
• Request a plain-English explanation for any pattern you plan to keep