Regex for HTTP 4xx (client error)
400-series client error status codes.
The HTTP 4xx (client error) regex is ^4\d{2}$ — copy it below, or open it in the explainer for a token-by-token breakdown.
The pattern
^4\d{2}$
What it matches
400401403404429
What it doesn't match
200301500
Notes & gotchas
400-499 represent client errors. 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, 429 Too Many Requests are common.
Code in your language
Use the explainer's Code tab to generate ready-to-paste snippets in JavaScript, Python, Java, .NET, Go, Ruby, and PHP for this pattern.
Open in explainer →Token-by-token breakdown
Every part of the pattern, left to right:
| Token | Meaning |
|---|---|
^ | start of string (or line in multiline mode) |
4 | literal text “4” |
\d{2} | exactly 2 times: digit (0–9) |
$ | end of string (or line in multiline mode) |
About this pattern
Developer-oriented patterns help with code analysis, log parsing, and data transformation. Use these in build scripts, CI tools, IDE search-and-replace, and code review utilities.
Quick usage in different languages
This exact pattern — with the correct escaping and idioms for each language:
- JavaScript:
/^4\d{2}$/.test(value) - Python:
re.match(r"^4\d{2}$", value) - Java:
Pattern.compile("^4\\d{2}$").matcher(value).matches() - C# / .NET:
Regex.IsMatch(value, @"^4\d{2}$") - Go:
regexp.MustCompile(`^4\d{2}$`).MatchString(value) - Ruby:
/^4\d{2}$/.match?(value) - PHP:
preg_match('~^4\d{2}$~', $value)
The explainer’s Code tab regenerates these for any pattern you paste, and the downloadable cheat sheet bundles the breakdown, all seven snippets, and the pitfalls below onto one printable page.
Common pitfalls
- Anchored to the whole string. This pattern uses ^ and $, so it requires the entire input to match. To find it inside a longer text, drop the anchors and use the global (g) flag.
- Escape it correctly per language. In Java and JavaScript strings each backslash must be doubled (\\d); in Python, Go, and C# use raw/verbatim strings so the backslashes survive.
- Validate beyond format. Matching the format doesn't guarantee the value is real. Confirm the http 4xx (client error) against a source of truth (database, API, or checksum) where it matters.
Standards & sources
This pattern is based on the following authoritative specification(s) and issuing authorities. Formats can change — always confirm against the primary source.
Related patterns
More patterns in the Developer category:
- HTTP 3xx (redirect)
- HTTP 5xx (server error)
- HTTP 2xx (success)
- URL query parameter
- HTTP method
- Windows environment variable
See also
Browse all 300 patterns in the library, or open this regex in the interactive explainer to see a token-by-token breakdown, test against custom input, and generate code in seven languages.