Extract pattern from a string

Question

In bash script, what is the easy way to extract a text pattern from a string?

For example, I want to extract X followed by 2 digits in the end of the st

Answer 1

There's a nifty =~ regex operator when you use double square brackets. Captured groups are made available in the $BASH_REMATCH array.

if [[ $STRING =~ (X[0-9]{2})$ ]]; then
    echo "matched part is ${BASH_REMATCH[1]}"
fi