— the whole value must be the number, nothing before or after.\n- `(?:\\+61 ?4|04)` — either `+61` (optional space) then `4`, or the local `04`. Australian mobiles start with 4 after the country code, and you drop the 0 when using +61.\n- `\\d{2}` — the next two digits (e.g. `12` in 0412).\n- ` ?\\d{3}` — optional space, three digits.\n- ` ?\\d{3}` — optional space, the last three digits.\n\n### Tests\n\n| Input | Should match | Does match |\n|---|---|---|\n| `0412 345 678` | ✅ | ✅ |\n| `0412345678` | ✅ | ✅ |\n| `+61 412 345 678` | ✅ | ✅ |\n| `+61412345678` | ✅ | ✅ |\n| `0412 345678` | ✅ | ✅ |\n| `(02) 9876 5432` | ❌ landline | ❌ |\n| `0412 345 67` | ❌ too short | ❌ |\n| `+61 0412 345 678` | ❌ extra 0 after +61 | ❌ |\n| `0512 345 678` | ❌ not a mobile prefix | ❌ |\n| `04123 45678` | ❌ odd grouping | ❌ |\n| `0412-345-678` | ❌ dashes | ❌ |\n| ` 0412 345 678` | ❌ leading space | ❌ (that's why the snippet uses `trim()`) |\n\n### Limits\n- **Dashes and brackets** aren't accepted. If users type `0412-345-678`, either strip non-digits first (`input.replace(/[\\s-]/g, '')`) and validate `^(?:\\+614|04)\\d{8} Write a regex from plain English with tests — CatchCase , or extend the pattern.\n- It checks the **format**, not whether the number exists.\n- Groupings other than 4-3-3 (like `04123 45678`) are rejected on purpose; relax the spaces to ` ?` between every digit if you want to accept any grouping.\n- For international numbers in general, use a library such as libphonenumber rather than a regex.","example_images":null}}
CatchCase

Write a regex from plain English with tests

Describe what to match and give examples. Get a regular expression for your language, explained piece by piece, with tests that should and should not pass.

How to use it

  1. Say where it will run JavaScript, Python, PCRE, .NET or a tool like VS Code search. Flavours differ.
  2. Give real examples Three strings that must match and three that must not. Edge cases are where regexes break.
  3. Run the tests Paste the pattern and the test list into regex101.com (pick your flavour) before using it.
  4. Prefer a parser when it's complex For emails, HTML or dates with real rules, use a library — the answer will say when.

The prompt

You are a careful developer. Write a regular expression for me.

Where it will run (language or tool and version): [REGEX_FLAVOUR]

What it should match, in plain words: [WHAT_TO_MATCH]

Examples that must match: [MATCH_EXAMPLES]

Examples that must not match: [NON_MATCH_EXAMPLES]

Return:

- The regex, with the flags, as a code block, plus a code snippet showing how to use it in my language.
- A piece-by-piece explanation.
- A test table of at least 10 strings — my examples plus edge cases you think of — with "should match" and "does match" columns. Check each one carefully against the pattern; if any fails, fix the pattern before answering.
- Limits: what it deliberately does not handle, and whether a parser or library would be safer for this job.

Prefer readable patterns over clever ones. Anchor the pattern if it validates a whole value; avoid patterns that risk catastrophic backtracking.

Example result

The pattern

^(?:\+61 ?4|04)\d{2} ?\d{3} ?\d{3}$

const AU_MOBILE = /^(?:\+61 ?4|04)\d{2} ?\d{3} ?\d{3}$/;

AU_MOBILE.test(input.trim()); // true or false

Piece by piece

  • ^ … $ — the whole value must be the number, nothing before or after.
  • (?:\+61 ?4|04) — either +61 (optional space) then 4, or the local 04. Australian mobiles start with 4 after the country code, and you drop the 0 when using +61.
  • \d{2} — the next two digits (e.g. 12 in 0412).
  • ?\d{3} — optional space, three digits.
  • ?\d{3} — optional space, the last three digits.

Tests

InputShould matchDoes match
0412 345 678✅✅
0412345678✅✅
+61 412 345 678✅✅
+61412345678✅✅
0412 345678✅✅
(02) 9876 5432❌ landline❌
0412 345 67❌ too short❌
+61 0412 345 678❌ extra 0 after +61❌
0512 345 678❌ not a mobile prefix❌
04123 45678❌ odd grouping❌
0412-345-678❌ dashes❌
0412 345 678❌ leading space❌ (that's why the snippet uses trim())

Limits

  • Dashes and brackets aren't accepted. If users type 0412-345-678, either strip non-digits first (input.replace(/[\s-]/g, '')) and validate ^(?:\+614|04)\d{8}$, or extend the pattern.
  • It checks the format, not whether the number exists.
  • Groupings other than 4-3-3 (like 04123 45678) are rejected on purpose; relax the spaces to ? between every digit if you want to accept any grouping.
  • For international numbers in general, use a library such as libphonenumber rather than a regex.

Related cases