CODEWITHSUNDEEP
utf-8lexical-analysisflex-lexer
Martin Cote
Martin Cote

Reputation: 29892

How to make a flex (lexical scanner) to read UTF-8 characters input?

It seems that flex doesn't support UTF-8 input. Whenever the scanner encounter a non-ASCII char, it stops scanning as if it was an EOF.

Is there a way to force flex to eat my UTF-8 chars? I don't want it to actually match UTF-8 chars, just eat them when using the '.' pattern.

Any suggestion?

EDIT

The most simple solution would be:

ANY [\x00-\xff]

and use 'ANY' instead of '.' in my rules.

Upvotes: 4

Views: 8055

Answers (2)

kiloalphaindia
kiloalphaindia

Reputation:

writing an negatet characterclass might also help:

[\n \t] return WHITESPACE; [^\n \t] retrun NON_WHITESPACE

Upvotes: 1

Aiden Bell
Aiden Bell

Reputation: 28384

I have been looking into this myself and reading the Flex mailing list to see if anyone thought about it. To get Flex to read unicode is a complex affair ...

UTF-8 encoding can be done, and most other encodings (the 16s) will lead to massive tables driving the automata.

A common method so far is:

What I did was simply write patterns that match single UTF-8 characters. They look something like the following, but you might want to re-read the UTF-8 specification because I wrote this so long ago.
You will of course need to combine these since you want unicode strings, not just single characters.

UB [\200-\277] %% 
[\300-\337]{UB}                   { do something } 
[\340-\357]{UB}{2}                { do something } 
[\360-\367]{UB}{3}                { do something } 
[\370-\373]{UB}{4}                { do something } 
[\374-\375]{UB}{5}                { do something }

Taken from the mailing list.

I may look at creating a proper patch for UTF-8 support after looking at it further. The above solution seems unmaintainable for large .l files. And is really ugly! You could use ranges similar to create a '.' substitute rule to match all ASCII and UTF-8 characters, but still rather ugly.

hope this helps!

Upvotes: 7

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