Pattern

Regex for File extension

Match the extension at the end of a filename.

The File extension regex is \.([a-zA-Z0-9]+)$ — copy it below, or open it in the explainer for a token-by-token breakdown.

The pattern

\.([a-zA-Z0-9]+)$

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What it matches

document.pdf
archive.tar.gz
image.PNG
script.sh

What it doesn't match

no-extension
.hiddenfile

Notes & gotchas

Captures just the last extension in group 1. For multi-part extensions like .tar.gz, you'll need a custom approach.

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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Token-by-token breakdown

Every part of the pattern, left to right:

Token	Meaning
`\.`	literal “.”
`(`	start of a capturing group
`[a-zA-Z0-9]+`	one or more: any of: lowercase letters, uppercase letters, digits
`)`	end of group
`$`	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, and code review utilities.

Quick usage in different languages

This exact pattern — with the correct escaping and idioms for each language:

JavaScript: /\.([a-zA-Z0-9]+)$/.test(value)
Python: re.match(r"\.([a-zA-Z0-9]+)$", value)
Java: Pattern.compile("\\.([a-zA-Z0-9]+)$").matcher(value).matches()
C# / .NET: Regex.IsMatch(value, @"\.([a-zA-Z0-9]+)$")
Go: regexp.MustCompile(`\.([a-zA-Z0-9]+)$`).MatchString(value)
Ruby: /\.([a-zA-Z0-9]+)$/.match?(value)
PHP: preg_match('~\.([a-zA-Z0-9]+)$~', $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 $, so it requires the entire input to match. To find it inside a longer text, drop the anchors and use the global (g) flag.
ASCII letters only. [a-zA-Z] excludes accented and non-Latin letters (é, ü, ß, ñ, and non-Latin scripts). For international input use Unicode properties like \p{L} with the u 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 file extension against a source of truth (database, API, or checksum) where it matters.

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