File Extension Regex
Extracts and validates the file extension of a filename, requiring at least one character before the final dot.
Regex Pattern
^.+\.([a-zA-Z0-9]+)$Pattern Breakdown
Hover over a token to see what it does.
| Token | Meaning |
|---|---|
| ^ | Anchors the match to the start of the string |
| .+ | Matches one or more of any character (the filename stem), greedily |
| \. | Matches a literal dot separating the stem from the extension |
| ( | Opens a capturing group for the extension itself |
| [a-zA-Z0-9]+ | Matches one or more letters or digits, the extension characters |
| ) | Closes the capturing group so the extension can be extracted |
| $ | Anchors the match to the end of the string |
Detailed Explanation
What it does
This pattern matches a filename that ends in a dot followed by an alphanumeric extension, capturing the extension in group 1. Because .+ is greedy, it matches up to the last dot in the filename, so multi-dot names like archive.tar.gz correctly extract gz rather than tar.gz.
Why it works
The leading .+ requires at least one character before the dot, which is what excludes dotfiles like .gitignore that have no filename stem before their leading dot. Since .+ is greedy, the regex engine backtracks from the end of the string until it finds the rightmost dot that still allows the trailing [a-zA-Z0-9]+ to match, guaranteeing the final extension is the one captured.
Common use cases
- Extracting a file extension to route uploads to the right handler
- Validating that a filename has an extension before allowing an upload
- Filtering directory listings for files of a particular type
- Building a MIME-type lookup keyed off the captured extension
Edge cases
- Multi-part extensions like archive.tar.gz only capture the final segment, gz, not tar.gz
- Dotfiles like .gitignore or .env are rejected because .+ needs at least one character before the dot
- A trailing dot with nothing after it, like notes., is rejected since [a-zA-Z0-9]+ requires at least one character
- Extensions with unusual characters, like a version tag file.v1.2, still match because digits are allowed in the extension class
Limitations
- Does not validate the extension against a known whitelist of real file types
- Extensions containing characters outside [a-zA-Z0-9], such as a stray hyphen, will not match
- Does not account for case sensitivity conventions of specific filesystems
Interactive Tester
Edit the pattern or text below — matching runs live in your browser.
%24&f=g&t=document.pdf%0Aarchive.tar.gz%0Aphoto.JPEG){kind=link}
Test Cases
Editable — add your own inputs to see if they pass.
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Language Variants
Production-ready examples in 12 languages.
const fileExtRegex = /^.+\.([a-zA-Z0-9]+)$/;
const match = 'archive.tar.gz'.match(fileExtRegex);
console.log(match?.[1]); // 'gz'Common Mistakes
Using a lazy .+? before the dot, which grabs the first dot instead of the last in multi-dot filenames
Fix: Keep .+ greedy so it backtracks to the final dot, correctly capturing gz from archive.tar.gz
Forgetting the leading .+ requirement, causing dotfiles like .gitignore to be treated as having an extension
Fix: Require at least one character before the dot with .+\. so dotfiles are excluded
Restricting the extension class to letters only, rejecting valid numeric extensions like .mp3 or .7z
Fix: Use [a-zA-Z0-9]+ to allow both letters and digits in the extension
Performance Notes
- The greedy .+ can cause some backtracking on long filenames, but since the input is typically short this is negligible
- Anchoring with ^ and $ prevents partial matches inside a longer string
- For extension extraction only (no full-string validation), a non-anchored /\.([a-zA-Z0-9]+)$/ is slightly cheaper
Browser Compatibility
| Engine | Supported | Notes |
|---|---|---|
| Chrome | Yes | — |
| Firefox | Yes | — |
| Safari | Yes | — |
| Edge | Yes | — |
| Node.js | Yes | — |