Regex to Match a File Extension
Copy the pattern, paste your filenames below, and see the extension on each one. Below the tester: how to grab just the last extension versus a double extension like .tar.gz.
What this pattern matches
This expression captures the final extension of a filename — the dot and the letters or digits after it, such as .pdf or .gz. With the multiline flag it works line by line on a list of files.
How it works, part by part
\. — a literal dot.
[A-Za-z0-9]+ — one or more letters or digits, the extension itself.
$ — the end of the line, so only the last extension is taken.
Single versus double extensions
For archive.tar.gz this pattern returns .gz, the true final extension. To capture a compound extension like .tar.gz, use \.[A-Za-z0-9]+(?:\.[A-Za-z0-9]+)?$. Note that a dotfile such as .gitignore is a name, not an extension — anchor on a non-dot character first if you need to exclude those.
Use it in your code
import re
text = "report.pdf\narchive.tar.gz\nphoto.JPEG\nMakefile\n.gitignore"
pattern = r'''\.[A-Za-z0-9]+$'''
for m in re.finditer(pattern, text, re.M):
print(m.group())const text = "report.pdf\narchive.tar.gz\nphoto.JPEG\nMakefile\n.gitignore";
const re = new RegExp(String.raw`\.[A-Za-z0-9]+$`, "gm");
console.log(text.match(re));FAQ
Does it treat .gitignore as an extension?
As written, yes, because it only looks for a dot and letters at the end. Require a character before the dot, like \w\.[A-Za-z0-9]+$, to skip dotfiles.
How do I get .tar.gz as one extension?
Use \.[A-Za-z0-9]+(?:\.[A-Za-z0-9]+)?$ to optionally include a second dotted segment.
Is the match case-sensitive?
The class covers both cases, so .JPEG and .jpeg both match. Lowercase the result in code if you need to compare.