Question

75.122.1.23

I need to replace the third number with 2 so it looks like 75.122.2.23

What regex can match the third section?

Answer 1

int replacementNumber = 2;
ip = ip.replaceAll("(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})", "$1.$2."+replacementNumber+".$4");

Commented form:

/^                 # match beginning
    (
        \d{1,3}    # one address segment
        \.         # separation dot
        \d{1,3})   # second address segment
    \.             # separation dot
    \d{1,3}        # address segment to be replaced
    \.             # separation dot
    (\d{1,3})      # last address segment
$/x                # match end, ignore comments