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From: Eric Blake <ebb9 AT byu DOT net>
Subject:  Re: Cygwin bash regexp matching doesn't treat "\b" properly
Date: Tue, 24 Nov 2009 22:51:24 +0000 (UTC)
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Dave Korn <dave.korn.cygwin <at> googlemail.com> writes:

> 
> $ [[ "foo" =~ [[:\<:]]foo[[:\>:]] ]]; echo $?
> 0
> 
>   (Note that I had to backslash-escape the < and > there.  In other contexts
> that might not be needed.)

But here's something weird with how bash manages quoting inside [[ ]].  If you 
add a subexpression, you no longer need to quote < or >:

$ [[ foo =~ ([[:<:]]foo[[:>:]]) ]]; echo $?
0

With further experimentation, it turns out that cygwin's regex(3) does not 
understand [[:<:][:>:]] as a character class that accepts either direction of 
word boundary (for shame).  So, modulo the difference in the number of 
subexpressions, the closest representation of \b becomes:

([[:<:]]|[[:>:]])

and an expression to match words that either end in a or begin in b would be:

$ [[ ' b ' =~ ([a ]([[:<:]]|[[:>:]])[b ]) ]]; echo $?
0
$ [[ ' ab '  =~ ([a ]([[:<:]]|[[:>:]])[b ]) ]]; echo $?
1

which looks so much shorter as ([a ]\b[b ])

-- 
Eric Blake



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