Shell Command: =~. get match group value from regex

 files="*.jpg"

regex="[0-9]+_([a-z]+)_[0-9a-z]*"

#unquoted in order to allow the glob to expand
for f in $files 
do
    if [[ $f =~ $regex ]]
    then
        name="${BASH_REMATCH[1]}"
        echo "${name}.jpg"    # concatenate strings
        name="${name}.jpg"    # same thing stored in a variable
    else
        echo "$f doesn't match" >&2 
        # this could get noisy if there are a lot of non-matching files
    fi
done     
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It's better to put the regex in a variable. Some patterns won't work if included literally.
This uses =~ which is Bash's regex match operator. 
The results of the match are saved to an array called $BASH_REMATCH. 
The first capture group is stored in index 1, the second (if any) in index 2, 
etc. Index zero is the full match.
refs: https://stackoverflow.com/questions/1891797/capturing-groups-from-a-grep-regex
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