Base64 Validator

What makes Base64 valid?

• Only characters from the alphabet of the chosen standard (for example A–Z a–z 0–9 + /).
• Correct padding: the length is a multiple of 4 and ends with at most two =. Base64URL and IMAP usually omit padding.
• No spaces or line breaks, except in MIME (76 characters per line) and PEM (64 characters per line).
• The length is never 4n+1, because one leftover character cannot represent a full byte.

Validate Base64 in code

// JavaScript
const isBase64 = (s) => /^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/.test(s);

# Python
import base64, binascii
def is_base64(s: str) -> bool:
    try:
        base64.b64decode(s, validate=True)
        return True
    except binascii.Error:
        return False