R5RS, 7.1.1 Lexical structure:
<identifier> --> <initial> <subsequent>* | <peculiar identifier>
<initial> --> <letter> | <special initial>
<letter> --> a | b | c | ... | z
<special initial> --> ! | $ | % | & | * | / | : | < | = | > | ? | ^ | _ | ~
<subsequent> --> <initial> | <digit> | <special subsequent>
<digit> --> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
<special subsequent> --> + | - | . | @
<peculiar identifier> --> + | - | ...
<syntactic keyword> --> <expression keyword>
| else | => | define
| unquote | unquote-splicing
<expression keyword> --> quote | lambda | if
| set! | begin | cond | and | or | case
| let | let* | letrec | do | delay
| quasiquote
