Re: Can I test if a parameter expansion has worked

[zzapper <zsh@rayninfo.co.uk>]

On 15/09/2022 09:24, Peter Stephenson wrote:
>> On 15/09/2022 08:34 zzapper <zsh@rayninfo.co.uk> wrote:
>> # adding a prefix '_s' to an image name
>>
>> f=stuff.jpg;
>>
>> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
>>
>> stuff_s.jpg  # good this is what i want
>>
>> # now pass 'bad' input
>>
>> f=stuff.txt;
>>
>> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
>> stuff.txt  #  expansion has obviously left the input unchanged but can I
>> test for that???
> No, there's no way of telling if some expansion actually changed an
> expression without actually testing the final result.  The differences
> between the cases (changed / not changed) are buried deep within the
> shell and don't leave an obvious trace above --- and in any case given
> how complicated zsh expressions can get finding a useful binary way of
> testing what the whole expression did would be incredibly hairy.
>
> Just test the resulting expression.
>
> pws
>

I now realise that it would horribly subjective to decide whether or not 
an expansion had succeeded in any case :)

f=pws

echo ${f/%(#m)pws/thanks $MATCH}


zzapper