How to implement case insensitive lexical parser in Golang using gocc? - go

Question

I need to build a lexical analyzer using Gocc, however no option to ignore case is mentioned in the documentation and I haven't been able to find anything related. Anyone have any idea how it can be done or should I use another tool?
/* Lexical part */
_digit : '0'-'9' ;
int64 : '1'-'9' {_digit} ;
switch: 's''w''i''t''c''h';
while: 'w''h''i''l''e';
!whitespace : ' ' | '\t' | '\n' | '\r' ;
/* Syntax part */
<<
import(
"github.com/goccmack/gocc/example/calc/token"
"github.com/goccmack/gocc/example/calc/util"
)
>>
Calc : Expr;
Expr :
Expr "+" Term << $0.(int64) + $2.(int64), nil >>
| Term
;
Term :
Term "*" Factor << $0.(int64) * $2.(int64), nil >>
| Factor
;
Factor :
"(" Expr ")" << $1, nil >>
| int64 << util.IntValue($0.(*token.Token).Lit) >>
;
For example, for "switch", I want to recognize no matter if it is uppercase or lowercase, but without having to type all the combinations. In Bison there is the option % option caseless, in Gocc is there one?

Answer 1

Looking through the docs for that product, I don't see any option for making character literals case-insensitive, nor do I see any way to write a character class, as in pretty well every regex engine and scanner generator. But nothing other than tedium, readability and style stops you from writing
switch: ('s'|'S')('w'|'W')('i'|'I')('t'|'T')('c'|'C')('h'|'H');
while: ('w'|'W')('h'|'H')('i'|'I')('l'|'L')('e'|'E');
(That's derived from the old way of doing it in lex without case-insensitivity, which uses character classes to make it quite a bit more readable:
[sS][wW][iI][tT][cC][hH] return T_SWITCH;
[wW][hH][iI][lL][eE] return T_WHILE;
You can come closer to the former by defining 26 patterns:
_a: 'a'|'A';
_b: 'b'|'B';
_c: 'c'|'C';
_d: 'd'|'D';
_e: 'e'|'E';
_f: 'f'|'F';
_g: 'g'|'G';
_h: 'h'|'H';
_i: 'i'|'I';
_j: 'j'|'J';
_k: 'k'|'K';
_l: 'l'|'L';
_m: 'm'|'M';
_n: 'n'|'N';
_o: 'o'|'O';
_p: 'p'|'P';
_q: 'q'|'Q';
_r: 'r'|'R';
_s: 's'|'S';
_t: 't'|'T';
_u: 'u'|'U';
_v: 'v'|'V';
_w: 'w'|'W';
_x: 'x'|'X';
_y: 'y'|'Y';
_z: 'z'|'Z';
and then explode the string literals:
switch: _s _w _i _t _c _h;
while: _w _h _i _l _e;