Pattern

Regex for HTTP 4xx (client error)

400-series client error status codes.

The pattern

^4\d{2}$

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What it matches

400
401
403
404
429

What it doesn't match

200
301
500

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.

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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

Once you've validated a candidate value matches this pattern, you'll typically use it inside your application code. Each language has its own regex syntax:

JavaScript: new RegExp(pattern).test(value) or /pattern/.test(value)
Python: re.match(pattern, value) with raw strings: r"pattern"
Java: Pattern.compile(pattern).matcher(value).matches()
C# / .NET: Regex.IsMatch(value, pattern)
Go: regexp.MustCompile(pattern).MatchString(value) — Go uses RE2 so some advanced features aren't available
Ruby: value =~ /pattern/ or pattern.match?(value)
PHP: preg_match('/pattern/', $value)

The explainer's Code tab generates these for any pattern you paste — including the right escaping and idioms for each language.

Common pitfalls

Anchors matter. The pattern starts with ^ and ends with $ — it expects the entire input to match. To find this pattern inside a longer text, remove the anchors and use the /g flag for multiple matches.
Case sensitivity. Letter ranges like [A-Z] only match uppercase. Use the i flag or [A-Za-z] for case-insensitive matching.
Escape user input. If you're building a regex from a string variable, escape regex metacharacters first to avoid bugs or injection — use RegExp.escape-equivalents in your language.
Performance. For this specific pattern the risk is low, but be cautious of nested quantifiers when adapting it — they can cause exponential backtracking on adversarial input.