Function lex 

pub(super) fn lex(
    version: FormatDescriptionVersion,
    input: &[u8],
) -> Lexed<impl Iterator<Item = Result<Token<'_>, Error>>> 

Available on (crate features formatting or parsing) and crate feature alloc only.

Parse the string into a series of Tokens.

version controls the version of the format description that is being parsed.

• When version is 1, [[ is the only escape sequence, resulting in a literal [. For the start of a nested format description, a single [ is used and is never part of the escape sequence. For example, [optional [[day]]] will lex successfully, ultimately resulting in a component named optional with the nested component day.
• When version is 2 or 3, all escape sequences begin with \. The only characters that may currently follow are \, [, and ], all of which result in the literal character. All other characters result in a lex error.