Regex to Match Basic Authentication Headers

This regex matches HTTP Basic Authentication credentials as transmitted in the Authorization header. Here is the token-by-token breakdown:

Basic — Matches the literal string Basic, identifying the HTTP Basic authentication scheme. This is case-sensitive as defined in the HTTP specification.

\s+ — Matches one or more whitespace characters between the scheme identifier and the encoded credentials.

( — Opens a capturing group for the Base64-encoded credentials.

[A-Za-z0-9+/]* — Matches zero or more characters from the Base64 alphabet: uppercase letters, lowercase letters, digits, plus signs, and forward slashes. The credentials are the Base64 encoding of the string username:password.

={0,2} — Matches zero to two equals signs for Base64 padding. Base64 encoding pads the output with equals signs to make the length a multiple of four characters. Zero, one, or two padding characters may be needed.

) — Closes the capturing group.

The g flag enables global matching. HTTP Basic Authentication is one of the simplest authentication mechanisms. The client sends the username and password as a Base64-encoded string in the Authorization header: Authorization: Basic dXNlcjpwYXNz (which decodes to user:pass).

This pattern is useful for identifying Basic auth headers in HTTP logs, security scanning tools, proxy servers, and authentication middleware. The captured group can be decoded to extract the username and password. Note that Basic auth transmits credentials in a reversible encoding (not encrypted), so it should only be used over HTTPS connections.