A-Z and a-z, without diacritics
0
or 1
any 7-bit US-ASCII character, excluding NUL
carriage return
Internet standard newline
controls
0
to 9
double quote ("
)
hexadecimal digit, case insensitive
horizontal tab
linefeed
linear white space.
linear white space.
Use of this rule permits lines containing only white space that are no longer legal in mail headers and have caused interoperability problems in other contexts.
Do not use when defining mail headers and use with caution in other contexts.
8 bits of data
space
visible (printing) characters
white space (space or horizontal tab)
Parsers for the common rules of RFC5234. These rules are referenced by several RFCs.
https://tools.ietf.org/html/rfc5234