[tpm] New Regex question - partially solved

Uri Guttman uri at stemsystems.com
Sun Oct 28 13:59:47 PDT 2012

On 10/28/2012 12:35 PM, Rob Janes wrote:
> Sigh.
> Regarding the accusation that I did not read the docs.  Here's an excerpt
> from man perlre (the highlights are mine):
>     Warning on \1 Instead of $1
>         Some people get too used to writing things like:
>             $pattern =~ s/(\W)/\\\1/g;
>         This is *grandfathered* (for \1 to \9) for the RHS of a substitute
> to avoid shocking the sed addicts, but it's

i said it was allowed but not considered good code.

>         a dirty habit to get into.  That's because in PerlThink, the
> righthand side of an "s///" is a double-quoted
>         string.  "\1" in the usual double-quoted string means a control-A.
> The customary Unix meaning of "\1" is
>         kludged in for "s///".  However, if you get into the habit of doing
> that, you get yourself into trouble if
>         you then add an "/e" modifier.
>             s/(\d+)/ \1 + 1 /eg;            # causes warning under -w
>         Or if you try to do
>             s/(\d+)/\1000/;
>         You can't disambiguate that by saying "\{1}000", whereas you can fix
> it with "${1}000".  The operation of
>         interpolation should not be confused with the operation of matching
> a backreference.  Certainly they mean
>         two different things on the left side of the "s///".
> So you see, \1 in the replace is not deprecated, it is grandfathered.
> Everybody, feel free to keep using it, although it's use may cause
> reviewers of your code to point and stare, perhaps gesticulate.

which is just as bad as being deprecated. i review code for my 
perlhunter business and that is something i will always note. there is 
NO reason to use \1 in the replacement when $1 is the same value and 
works correctly in all situations. just because something may work 
doesn't make it good code.

> stylistically, I'd say if you're using \1 on the match side, don't switch
> to $1 on the replace side.  It's confusing.

and that is way off base too. $1 is the standard way to access a grab 
both in the replacement and afterwards.
> As for being greatly mistaken about \1 and $1, I think not.  \1 and $1 are
> slightly different beasts on the match side of an s/// regex.  They both
> work, and are not deprecated.  On the replace side, \1 and $1 are
> equivalent, there is no deprecation of \1.  I've made specific statements
> about how \1 and $1 behave on the match side so as to better understand
> their differentiation.  I've posted tested examples as well, and snippets
> of the documentation.  So, Uri, what exactly is it that I'm greatly
> mistaken about?

what you just said and what the docs suggest. you keep doing your style. 
it will not help you get a better perl job if you are stubborn about 
improving your style. and yes, style matters a great deal in quality code.


More information about the toronto-pm mailing list