How do I test this regex in JavaScript and TypeScript?

Use regex.test(): const regex = /\.([a-zA-Z0-9]+)(?:\?|#|$)/i; const isValid = regex.test(inputString);

What is an example of a matching and non-matching string?

A valid matching string is "/uploads/invoice_report.pdf?version=2". A non-matching string is "folder_without_extension/".

Is this regex safe from ReDoS (Catastrophic Backtracking)?

Yes, this pattern is optimized for linear execution time and safe for browser-side validation without locking up the UI thread.

HomeRegex LibraryFile Extension Extraction Regex

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File Extension Extraction Regex

Isolates the trailing file extension from local paths, URLs, or document names while safely ignoring query strings and hashes.

Regular Expression Pattern:

/\.([a-zA-Z0-9]+)(?:\?|#|$)/i

Valid Match Example:

/uploads/invoice_report.pdf?version=2

Invalid / Non-Match Example:

folder_without_extension/

Regex Syntax Breakdown & Explanation

•\. : Literal period preceding extension name
•([a-zA-Z0-9]+) : Captures alphanumeric extension tag
•(?:\?|#|$) : Stops before URL query parameters, fragment anchors, or end of string

Implementation in Popular Languages

JavaScript / TypeScript

const regex = /\.([a-zA-Z0-9]+)(?:\?|#|$)/i;
const isValid = regex.test("/uploads/invoice_report.pdf?version=2");
console.log(isValid); // true

Python (re)

import re

pattern = r"\.([a-zA-Z0-9]+)(?:\?|#|$)"
match = re.match(pattern, "/uploads/invoice_report.pdf?version=2")
print(bool(match)) # True

Go (regexp)

package main
import (
  "fmt"
  "regexp"
)

func main() {
  re := regexp.MustCompile(`\.([a-zA-Z0-9]+)(?:\?|#|$)`)
  fmt.Println(re.MatchString("/uploads/invoice_report.pdf?version=2"))
}

Frequently Asked Questions

Everything you need to know regarding specifications, syntax, and security best practices.

The regular expression pattern is /\.([a-zA-Z0-9]+)(?:\?|#|$)/i. Isolates the trailing file extension from local paths, URLs, or document names while safely ignoring query strings and hashes.