NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/lang/c/19098

Internet Message Format | 1992-12-31 | 2.0 KB

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From: [e-mail] (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: Wanted: program to extract comments from C
Date: 31 Dec 1992 23:18:36 GMT
Organization: Lawrence Berkeley Laboratory, Berkeley CA
Lines: 30
Message-ID: <[e-mail]>
References: <[e-mail]> <[e-mail]>
NNTP-Posting-Host: 128.3.112.15

In article <[e-mail]> [e-mail]
(Uniteq Application Systems) writes:
>... you could also do it with a lot fewer lines (and a much
>more complicated engine, but we let lex worry about that stuff):
>
>%%
>"//".*                {printf("%s\n", yytext); /* C++ comments */ }
>"/*"([^*]|\*+[^*/])*\*+\/    {printf("%s\n", yytext); /* std C comments */ }
>.|\n                { /* ignore everything else */ }
>
>I am uncertain about the correct semantics when mixing the two commenting
>styles - the above code allows `//' to comment out a `/*' but not a `*/'.
>The code Kenneth presented took the rather strange (IMO) stance of making
>either type of comment able to override the other (`//' remains active
>within a `/* ... */' pair).

All of these points are valid, but there are two problems with this
approach.  The first is that lex (including its improved cousin flex)
has a fairly small limit on the size of `yytext'.  Comments may often
exceed this limit.  One can raise it in any lex program (simply
redefine YY_BUF_SIZE), but any limit may prove too small (I have seen
truly enormous comments in some code).  The second is that regular
expressions for recognizing /* ... */ comments are notoriously
difficult.  The one above appears correct, but anyone trying to
reproduce this from memory may get it wrong.  For these reasons it
usually seems better to use start states (easier in flex, with its
exclusive start states) to handle this.
--
In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE ([phone])
Berkeley, CA        Domain:    [e-mail]