Cut a glob from both ends of a string in one command without storing an intermediary value.

gistfile1.sh

#!/usr/bin/env bash
shopt -s extglob

a='aaabbbbccccc'

# Could be half this length if string assignment in an arithmetic context worked, or if the "foo" in ${#foo} could be any expression rather than a parameter.
echo "${a:b=$(b=${a%${a##*(a)}}; echo ${#b}),b:$(c=${a%%*(c)}; echo ${#c})-b}"

# Equivalent to any of:
# tmp="${a##*(a)}"; echo "${tmp%%*(c)}"
# [[ $a =~ a*(b*)c* ]] && echo "${BASH_REMATCH[1]}"

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