Actually, regarding shells, the POSIX people pretend that it is still 1988.
The POSIX shell is the Bourne shell (1979) plus half of the innovations brought by ksh88.
The other half of the improvements from ksh88 and all of the improvements from ksh93 are not included in the POSIX shell standard.
Also for POSIX compliant shells one must not forget that also the other programs invoked by the shell must not be used with non-POSIX options.
For example, in a bash script I used the ksh93 syntax:
${VARIABLE:64:32}
Then I wanted to make it POSIX compliant and I rewrote it in the longer:
$(expr substr "${VARIABLE}" 65 32)
But then I discovered that POSIX expr lacks substr, so I had to rewrite the expression into the even longer:
$(awk -v s="${VARIABLE}" 'BEGIN {print substr(s, 65, 32)}')
which is the only POSIX compliant way to extract substrings.
In most cases the rational way is to write scripts for bash or zsh, which include all the ksh93 features plus brace expansion (from csh, then enhanced by zsh).
Writing POSIX compliant scripts results in much longer scripts in which the chances of bugs are much higher.
