Regex Pattern Builder and Tester

Generate, explain, and test regular expression patterns for common and complex text matching scenarios with step-by-step breakdowns and edge case handling.

Prompt Template

You are a regex expert. Help me build and understand a regular expression pattern.

**What I need to match:**
[Describe what you want to match — e.g., "Extract all email addresses from a block of text" or "Validate phone numbers in international format" or "Parse CSV fields that may contain quoted commas"]

**Programming language/engine:**
[e.g., JavaScript, Python, PCRE, .NET, Go]

**Sample input text:**
```
[Paste sample text here]
```

**Expected matches:**
[List what should match and what should NOT match]

Please provide:
1. **The regex pattern** with appropriate flags
2. **Step-by-step breakdown** — explain each component of the pattern in plain English
3. **Visual railroad diagram** description of the pattern flow
4. **Test cases table** — 10+ test strings showing match/no-match with the captured groups
5. **Edge cases** — tricky inputs that might break the pattern and how to handle them
6. **Performance notes** — any backtracking risks, and optimization suggestions
7. **Code snippet** — ready-to-use implementation in the specified language
8. **Common variations** — 2-3 alternative patterns for stricter or looser matching

Example Output

# Regex: Email Address Extractor

Pattern

[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}

Flags: `g` (global), `i` (case-insensitive)

Step-by-Step Breakdown

| Component | Meaning |

|-----------|--------|

| `[a-zA-Z0-9._%+-]+` | Local part: one or more alphanumeric chars, dots, underscores, percent, plus, hyphen |

| `@` | Literal @ symbol |

| `[a-zA-Z0-9.-]+` | Domain: one or more alphanumeric chars, dots, hyphens |

| `\.` | Literal dot before TLD |

| `[a-zA-Z]{2,}` | TLD: two or more letters (com, org, co.uk) |

Test Cases

| Input | Match? | Captured |

|-------|--------|----------|

| `user@example.com` | ✅ | `user@example.com` |

| `first.last+tag@sub.domain.co.uk` | ✅ | `first.last+tag@sub.domain.co.uk` |

| `user@.com` | ❌ | — |

| `@example.com` | ❌ | — |

| `user@example` | ❌ | — |

| `user name@example.com` | ❌ (space) | — |

JavaScript Implementation

const emailRegex = /[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/gi;

const matches = text.match(emailRegex) || [];

console.log(`Found ${matches.length} emails:`, matches);

Edge Cases

- **Quoted local parts** (`"user name"@example.com`): Valid per RFC 5321 but rare — add `"[^"]+"` alternative if needed

- **IP domain** (`user@[192.168.1.1]`): Not matched by default — extend domain part if required

- **Internationalized emails** (`user@例え.jp`): Requires Unicode pattern support

Tips for Best Results

💡Always specify the regex engine — JavaScript, Python, and PCRE have different feature support
💡Test with at least 10 edge cases including empty strings, special characters, and Unicode
💡Avoid catastrophic backtracking by using possessive quantifiers or atomic groups when available
💡Use named capture groups (?P<name>...) in Python or (?<name>...) in JS for readable code
💡For production validation, prefer dedicated libraries over regex (e.g., email-validator for emails)

Try it with

ChatGPT Claude Gemini

Frequently Asked Questions

What is the Regex Pattern Builder and Tester prompt?

Generate, explain, and test regular expression patterns for common and complex text matching scenarios with step-by-step breakdowns and edge case handling. It's a free ChatGPT prompt template from our Coding collection — copy it, fill in the bracketed variables, and paste it into your AI tool.

Which AI tools work with this prompt?

It's written and tested for ChatGPT, Claude and Gemini. Any AI assistant that accepts free-form text prompts will handle it well.

How do I customize this ChatGPT prompt?

Replace the bracketed variables — such as [e.g., JavaScript, Python, PCRE, .NET, Go], [Paste sample text here] — with your own details before running it. Always specify the regex engine — JavaScript, Python, and PCRE have different feature support

Is this prompt free to use?

Yes. Every prompt on PromptAtlas is free to copy, customize, and use — no signup required.

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