RegEx for detecting base64 encoded strings

Question

I need to detect strings with the form @base64 (e.g. @VGhpcyBpcyBhbiBlbmNvZGVkIHN0cmluZw==) in my application.

The @ has to be at the begin

Answer 1

Here's an alternative regular expression:

^@(?=(.{4})*$)[A-Za-z0-9+/]*={0,2}$

It satisfies the following conditions:

• The string length after the @ sign must be a multiple of four - (?=^(.{4})*$)
• The content must be alphanumeric characters or + or / - [A-Za-z0-9+/]*
• It can have up to two padding (=) characters on the end - ={0,2}