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Re: 'expr' and weird character classes
From: |
Andreas Schwab |
Subject: |
Re: 'expr' and weird character classes |
Date: |
Mon, 10 May 2004 11:53:40 +0200 |
User-agent: |
Gnus/5.1006 (Gnus v5.10.6) Emacs/21.3.50 (gnu/linux) |
DervishD <address@hidden> writes:
> Hello all :)
>
> I'm using coreutils 5.2.1 and I've noticed a weird behaviour of
> 'expr'. I don't know if it's my fault, but a command like:
>
> $ expr "x" : "[:alpha:]"
This matches "x" against the set of characters ":ahlp".
> returns 0 instead of 1, but a command like:
>
> $ expr "x" : "[[:alpha:]]"
This matches "x" against the set of characters from the character class
"alpha".
> returns 1, correctly. That is, 'expr' understands character
> classes but it needs that double bracket :(
The syntax of a character class is [:CLASS:], _including_ the brackets.
Thus to build a regular expression that matches a single lower case letter
or digit or underscore, you write "[[:lower:][:digit:]_]".
> This is non portable AFAIK, at least is not SUSv3 compliant regarding
> basic regular expressions
Please read again.
> and other binaries like 'tr'
`tr' doesn't expect regular expressions as arguments, but rather they are
interpreted as character sets without the surrounding brackets.
> Am I doing something wrong?
You are mixing the syntax for a regular expression containing a character
set with the syntax for a character class inside a character set.
Andreas.
--
Andreas Schwab, SuSE Labs, address@hidden
SuSE Linux AG, Maxfeldstraße 5, 90409 Nürnberg, Germany
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"And now for something completely different."