Re: How make multifinished DFA for merged regexps?

Am 21.12.2019 um 01:29 schrieb Andy:
> Greedy algorithms match longest regexp. For example operators "+" and "++",
> int numbers "123" and float numbers "123.456e3".
> On '.' will finish state of number, but we will inside automata for float
> number. But can be errors: after '.' will 'a'. We must backtrack to last
> finished state?


Why should "123." not form a valid float number? In fact it's the C way
to force a possibly int number into a float.


If your lexer requires backtracking, because it e.g. is LR(n), then this
is the only solution. Unlike parsers, which may work based on
shift/reduce actions, a scanner should be made simpler.


> I want avoid backtracking. Maybe after backtracking we must
> read chars from auxiliary token buffer instead of stream up to previous
> position? But this complicated parsing.


Parsers require a lookahead of at least one token. So scanners should
implement at least a lookahead of one character, depending on the
complexity or weirdness of a language definition.


DoDi

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