Piping to another process (like 'sed' and 'perl' suggested above) might be very expensive, especially when you need to run this operation multiple times. Bash does support regexp:
[[ "string" =~ regex ]]
Similarly to the way you extract matches in your favourite editor by using $1, $2, etc., Bash fills in the $BASH_REMATCH array with all the matches.
In your particular example:
str="first url1, second url2, third url3"
if [[ $str =~ (second )([^,]*) ]]; then
echo "match: '${BASH_REMATCH[2]}'"
else
echo "no match found"
fi
Output:
match: 'url2'
Specifically, =~ supports extended regular expressions as defined by POSIX, but with platform-specific extensions (which vary in extent and can be incompatible).
On Linux platforms (GNU userland), see man grep; on macOS/BSD platforms, see man re_format.
