Re: backreferences

[Ray Andrews <rayandrews@eastlink.ca>]

On 10/16/2015 05:35 AM, Bart Schaefer wrote:
> } when nested and even when 'doing something else'?
>
> Yes.

Ok, good to know.  I've only been using backreferences since two days 
ago and up till now it's seemed that you had to add parentheses to 
create the reference and existing parentheses only had their existing 
syntax.  I suppose this means that when you do have existing parentheses 
you'll get a 'match' whether you want one or not but so what, just  
ignore it.  Yup, that's best.
> ... but the part about not needing the
> middle * is wrong, because (^edcba) matches xxxxedcbaxxxx just fine, and
> I assume you don't want that.

    test2 ()
    {
    match=
    sstring="abcdeedcbaabcde"
    # Bart doesn't like:
    #if [[ "$sstring" = (#b)([(^(edcba))]*)(edcba)(*) ]];
    # Bart likes:
    if [[ "$sstring" = (#b)(^edcba)(edcba)(*) ]];

    then
       echo "\nIt's a poyfect match\n"
    fi

    echo "one   $match[1]"
    echo "two   $match[2]"
    echo "three $match[3]"
    echo "four  $match[4]"
    echo "five  $match[5]"
    }

    It's a poyfect match

    one   abcde
    two   edcba
    three abcde
    four
    five

... match[1]  seems to agree with your previous interpretation, no?


> This needs to be (#b)(^edcba*)(edcba)(*)

That produces identical output as well, so what's the diff? Probably one 
of those things that blows up in your face one day ...
> } God knows.  But your simplified command works fine too, and I'll
> } take it on faith.  I've never seen any sort of 'any number of characters'
> } sort  of thing look other than:
> } [....]*
>
> No, now you're confusing grep-style regular expressions with zsh patterns.

... which is what I meant to say.
>      EGREP	ZSH
>      .		?
>      .*		* or ?#
>      .+		?##
>      .?		(?|)
>      [xyz]	[xyz]
>      [xyz]*	[xyz]#
>
> There's a lot more but those are the most important bits.

A table like that is worth tattooing onto one's arm.  Seriously once a 
fella has learned a bit of regex it becomes burnt into the brain, and 
it's an act of deliberation to use the other syntax.  It sorta makes it 
worse that they are similar :(  Is a complete table available somewhere?
> } ... so you can see where I'd go astray there.  Ok, so
> } ^(edcba)
> } is individual character matches and
> } (^edcba)
> } is  anything up to "edcba"
>
> No.  [^edcba] is individual character matches and (^edcba) is anything
> other than the literal string edcba,

Ok, got it.  A mortal's guide to this stuff would sure be useful. All 
the docs tend to dive right in to the deep end and immediately start 
explaining all the possible obscure permutations when KSH_GLOB is set 
and it's not leap year but it IS a Friday.  Such control!  But we start 
with the basics.
> including longer strings that have
> edcba as a substring.  Negated patterns are really tricky.

Na.