parameter expansion pattern replacement does not match empty string at beginning/end

[dicktyr]

ksh ignores empty string pattern and does not perform substitution

% ksh -c 'x=a; echo ${x/#/.}'
a

by contrast bash matches and substitutes as expected

% bash -c 'x=a; echo ${x/#/.}'
.a

% ksh --version
  version         sh (AT&T Research) 93u+ 2012-08-01

[dannyweldon]

Agreed, and zsh behaves the same as bash as well.

[McDutchie]

Not sure if this is a bug or a legitimate implementation difference. Nothing in the ksh93 man page says that the pattern following # or % may be empty.

yash does not recognise an empty pattern there either.

[siteshwar]

It's just a difference between how shells behave differently on empty patterns like '#' and '%'. POSIX does not define how ${var/pattern/replacement} should work. I think we should close this issue as not a bug.

[McDutchie]

Yes. It could be a feature request instead of a bug. This does seems like a rather convoluted way of adding something to the beginning or end of a variable. You can simply string them together like echo .${a}. But scripters do strange things all the time, and better compatibilty with bash and zsh could be considered a reason to change this.

[kernigh]

bash and zsh don't agree about empty patterns. Given try.sh with

x=abcd
echo ${x/''/e}
echo ${x//''/e}

then

$ bash try.sh
abcd
abcd
$ zsh try.sh
eabcd
eaebeced

Here ksh acts like bash.

[McDutchie]

But the pattern reported was #, not empty.

$ cat try.sh
x=abcd
echo ${x/#/e}
echo ${x//#/e}
$ bash try.sh
eabcd
abcd
$ zsh try.sh
eabcd
eabcd

Meh. I'm with @siteshwar ... close as not a bug.