Validate Base64 using Notepad++

Notepad++ is the favourite editor of many Windows developers, and with a few regular expressions it becomes a handy Base64 checker, especially for large values that are awkward to paste into a website.

1. Find invalid characters

Open Search → Find (Ctrl+F), choose Search Mode: Regular expression, and search for:

[^A-Za-z0-9+/=\r\n]

Every match is a character that does not belong in standard Base64. Use Find All in Current Document to list them with line numbers. For Base64URL use [^A-Za-z0-9\-_=\r\n].

2. Check that “=” only appears at the end

=[^=\r\n]

Any match means padding was found in the middle of the value — usually two values were concatenated.

3. Remove line breaks

Many decoders reject line breaks. Replace (Ctrl+H) with regular expressions:

• Find: \R (any line ending)
• Replace with: (empty)

4. Validate the whole value

Select everything and search for this regex (single line, no breaks):

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

If the whole text is matched, the value is valid standard Base64. If not, the length or padding is wrong.

5. Encode and decode inside Notepad++

The MIME Tools plugin (installed by default in most versions, otherwise via Plugins → Plugins Admin) adds Plugins → MIME Tools → Base64 Encode / Base64 Decode. Select text and run the command. Note that it treats the text in the document’s current encoding, so set Encoding → UTF-8 first for non-English text.

When regex is not enough

Regular expressions cannot tell you where a value was cut off or whether the decoded data is a valid image. For that, use the online validator (shows positions of all problems), the repair tool, and the Base64 to File decoder (identifies the file type).