Quoted-Printable Decoder

Quoted-Printable is primarily used in email systems (MIME) for encoding message bodies. It is the Content-Transfer-Encoding method for text-based email content that contains non-ASCII characters.

A soft line break is represented by = at the end of a line followed by CRLF. It indicates that the line was wrapped for transmission but should be rejoined when decoded. This ensures lines stay under 76 characters.

Quoted-Printable keeps most ASCII text readable and only encodes non-ASCII or special characters. Base64 encodes everything and produces output about 33% larger. QP is preferred for mostly-ASCII content, while Base64 is better for binary data.

Each byte of the UTF-8 sequence becomes =XX: an e-acute is two bytes written =C3=A9, and a Chinese word becomes twelve =XX groups. ASCII letters, digits, and most punctuation pass through literally, which keeps European-language text mostly readable - the design goal of the encoding.

RFC 2045 section 6.7 requires encoded lines no longer than 76 characters. When a line must be split, an = at the end of the line marks a soft break - the decoder removes it and joins the lines with no space or newline. A hard line break in the original is preserved as =0D=0A, so the two are unambiguous.

As =3D - the escape character must escape itself, exactly like a backslash in string literals. Position disambiguates the trailing case: =3D is data, while a bare = at line end is transport (a soft break). This is also why a quoted-printable body where every = starts a valid hex pair decodes cleanly, and anything else signals corruption.

In encoded-words per RFC 2047: a Subject header like =?UTF-8?Q?Hello=20world?= where the Q signals quoted-printable inside a charset/encoding envelope. The body of a text/plain email commonly declares Content-Transfer-Encoding: quoted-printable directly. This tool decodes the raw body form as well as the Q-fragment form.

Telltale signs: sequences like =C3=A9 or =20 sprinkled through otherwise-readable text, lines ending in a bare =, and line lengths clustering at 76 characters. A body that is 90% printable ASCII with occasional =XX escapes - rather than Base64's uniform four-character alphabet - is almost certainly quoted-printable.