Question

I’m trying to parse lines of the form:

command arg1[ arg2, ... argn]

such as:

usemtl weasels

or

f 1/2/3 4/5/6 7/8/9

Here is my regex:

^(\\w+)(( \\S+)+)$

When I parse the line “usemtl weasels”, I get the following capture groups:

Match 0: 'usemtl weasels'
Match 1: 'usemtl'
Match 2: ' weasels'

Why the space before the second match group? It doesn’t show up in Rubular.

Answer 1

Grouping in java regex is a little strange. Group 0 gives you the complete match of your regex – this is the same in all regex implementations I know. But group n (for n >= 1) will give you the last match of the n th declared group, not the n th match found.

Your second match gives you ‘ weasels’ with a leading blank, because your pattern contains that blank. You declared your 2nd group (( \\S+)+) and this group gives you the second match.

If you apply your pattern to the string a b c d, your group 0 will be a b c d, group 1 will be a, group 2 will be b c d and group 3 will be d, because this is the last match of your 3rd declared (inner) group ( \\S+).