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

A valid matching string is "SGVsbG8gV29ybGQh". A non-matching string is "Invalid Base64 string==".

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.

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Base64 Encoded String Validation Regex

Q: How do I test this regex in JavaScript and TypeScript?

Use regex.test(): const regex = /^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/; const isValid = regex.test(inputString);

Validates standard RFC 4648 Base64 encoded strings with valid length multiples of 4 and padding.

Regular Expression Pattern:

/^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/

Valid Match Example:

SGVsbG8gV29ybGQh

Invalid / Non-Match Example:

Invalid Base64 string==

Regex Syntax Breakdown & Explanation

•(?:[A-Za-z0-9+/]{4})* : Matches multiples of 4 base64 characters
•(?:...==|...=)? : Validates 0, 1, or 2 trailing padding equals signs

Implementation in Popular Languages

JavaScript / TypeScript

const regex = /^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/;
const isValid = regex.test("SGVsbG8gV29ybGQh");
console.log(isValid); // true

Python (re)

import re

pattern = r"^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$"
match = re.match(pattern, "SGVsbG8gV29ybGQh")
print(bool(match)) # True

Go (regexp)

package main
import (
  "fmt"
  "regexp"
)

func main() {
  re := regexp.MustCompile(`^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$`)
  fmt.Println(re.MatchString("SGVsbG8gV29ybGQh"))
}

Frequently Asked Questions

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

The regular expression pattern is /^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/. Validates standard RFC 4648 Base64 encoded strings with valid length multiples of 4 and padding.