Order of precedence in lexer

Question

I am writing a lexer and a parser for Excel formulas.

In Excel, we could assign a cell a name. For example, abc is a valid name, whereas, it is forbidden to name a cell B2 to avoid the confusion with the cell B2. So once we meet a formula =B2, we are sure that B2 refers a cell rather than a user defined name.

In my lexer_formula.mll, I have defined identifiers:

let lex_cell = ['A' - 'Z']+ ['0' - '9']+ (* regular expressions to include all the cells *)
let lex_name = ['A' - 'Z' '0' - '9']+ (* regular expressions to include all the names *)

But a string like B2 with match both lex_cell and lex_name, does anyone know how I could tell the lexer to consider first lex_cell, then lex_name? Will it be sufficient to put lex_cell before lex_name in rule token = parse?

Are you actually using ocamllex? You have a lot of tags, but you don't ever say which scanner generator you are using. — rici, May 30 '16 at 04:11

score 0 · Accepted Answer · answered May 30 '16 at 05:50

According to the ocamllex manual, it's sufficient to put lex_cell first:

If several regular expressions match a prefix of the input, the “longest match” rule applies: the regular expression that matches the longest prefix of the input is selected. In case of tie, the regular expression that occurs earlier in the rule is selected.

Order of precedence in lexer

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