Re: Regular expression grammar?

Zalman Stern <zalman@netcom18.netcom.com>
24 Sep 1999 22:53:59 -0400

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Re: Regular expression grammar? cbarron3@ix.netcom.com (1999-09-27)
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From: Zalman Stern <zalman@netcom18.netcom.com>
Newsgroups: comp.compilers
Date: 24 Sep 1999 22:53:59 -0400
Organization: Netcom
References: 99-09-051 99-09-070
Keywords: lex, parse

lex@cc.gatech.edu wrote:
: %token SYMBOL OR STAR LPAREN RPAREN
: %%
: regexp : r1 ;


: r1 : r2 | r1 OR r2 ;
: r2 : r3 | r2 r3 ;
: r3 : primary | r3 STAR ;


: primary : parenthesized | SYMBOL ;




: parenthesized : LPAREN regexp RPAREN ;
: %%


This grammar allows "a**" which the grammar I sent does not. I believe
allowing "a**" is correct and not allowing it is incorrect given the
definition of regular expressions. But I cannot see a sequence of STAR
operators ever being different than a single one and am tempted to disallow
it as its probably a typo. (A smattering of UNIX tools I just tried do
allow "a**".)


Thanks to lex (the poster) for showing a more formal approach to solving
the problem. (Which even manages to avoid a bug!)


-Z-


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