Keep matching pattern in shell parameter expansion

Question

I can remove a pattern in a bash variable using ${variable##pattern} (leading) or ${variable%%pattern} (trailing).

But I can't find a bash-only way to keep the pattern and throw the rest.

I know there are solutions using sed, awk, or grep, but I want to know if there is a reasonably efficient bash-only solution that I am overlooking?

Answer 1

${var%"${var##pattern}"}
${var#"${var%%pattern}"}

Example:

$ k='ab*10cd20ef*'
$ echo "${k%"${k##*[0-9]}"}"
ab*10cd20
$ echo "${k#"${k%%[0-9]*}"}"
10cd20ef*

Note the quotes are important to prevent the shell from interpreting the expansions as a pattern. Try echo "${k#${k%%[0-9]*}}" to see it outputs an incorrect result.