^start of string
\s*any whitespace character, repeated any number of times
Group 3((([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])) Group 4(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3} {3}repeats the group contents below exactly 3 times:
Group 5([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) 0-9one character from 0 (4810) to 9 (5710)
2nd Alternative[1-9][0-9] 1-9one character from 1 (4910) to 9 (5710)
0-9one character from 0 (4810) to 9 (5710)
1the literal 1 (4910 / 618 / 3116)
{2}matches exactly 2 characters from the set below:
0-9one character from 0 (4810) to 9 (5710)
4th Alternative2[0-4][0-9] 2the literal 2 (5010 / 628 / 3216)
0-4one character from 0 (4810) to 4 (5210)
0-9one character from 0 (4810) to 9 (5710)
0-5one character from 0 (4810) to 5 (5310)
\.the literal . (4610 / 568 / 2E16)
Group 6([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) 0-9one character from 0 (4810) to 9 (5710)
2nd Alternative[1-9][0-9] 1-9one character from 1 (4910) to 9 (5710)
0-9one character from 0 (4810) to 9 (5710)
4th Alternative2[0-4][0-9] \s*any whitespace character, repeated any number of times
$end of string, or before its final line terminator