Standard Base64 (RFC 4648)

RFC 4648 section 4 defines the “base64” encoding that most programming languages implement by default: btoa(), PHP’s base64_encode(), Python’s b64encode(), Java’s Base64.getEncoder(), and so on.

Rules

• Alphabet: A–Z a–z 0–9 + / (see Base64 characters).
• Padding with = so the length is always a multiple of 4.
• No line breaks. RFC 4648 says implementations must not add line feeds unless a referring specification (like MIME) explicitly requires them.
• Decoders must reject characters outside the alphabet unless the referring specification says otherwise.
• Unused bits in the last character should be zero (canonical encoding); strict decoders may reject non-zero bits.

Validation regex

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

This matches every valid padded value, including the empty string. Check your value interactively with the Base64 validator.

Related

• Base64URL for URLs and tokens
• MIME for e-mail with line wrapping