> On 17 Feb 2019, at 17:36, Peng Yu <pengyu...@gmail.com> wrote:
>
> But how to recognize the nested parameter expansion assignment in the
> first place? The lexer should have builtin states to capture paired
> `{` `}`, and use states to remember whether it is in substring
> extraction or pattern replacement in order to make sure to capture any
> errors at the level of the lexer.
Such matched pairs can be recognized in the lexer by using an integer starting
at 0 adding 1 for each '{' and -1 for each '}' when valid. If one gets non-zero
at the end of the expression, there is a mismatch. The problem is how to
recognize the end of the expression. The Bison parser does that by a lookahead
token if needed. So that might suggest to put it on the parser.
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