String:
192.168.0.2
regular expression :
(\d+).(\d+).(\d+).(\d+)
substitution :
\1\2\3\4
result :
19216802
every ( ) represent a group, \d+ means any digits, all the line mean there are four groups between “.” and those group can only be digits.
subsitution simply show goup1 group2 group3 group4