Regex Capturing Group with alternative doesn't match

I have the following string where I want to match the valid <key>:<value> pairs.

A valid <key> is anything with a non-whitespace character followed by :
A valid <value> is either enclosed in [] or a string without whitespaces.

key1:value1 key#2:@value#2 nyet key3:[@value#3, value4] key4:[value5] :bar

Basically I want to match everything except nyet and :bar

I came up with following regex \S+:(\S+|\[[^]]+\]) but it doesn't seem to match the expression in key3:[@value#3, value4]. In the capturing group, the second alternative \[[^]]+\] should match this expression, so I don't understand why it doesn't match.

The following regex works: \S+:([^([ )]+|\[[^\]]+\]) but doesn't feel elegant.

Questions:

Why does the first regex \S+:(\S+|\[[^]]+\]) not work?
How would a more elegant solution look to match the key value pairs?

Solution

In the pattern you can switch the alternatives \S+:(\[[^]]+\]|\S+) but is would also match the [] in that case.

You could also exclude matching the : in the first part [^\s:]+:(\[[^]]+]|\S+) using a negated character class.

For the groups, you could use an alternation and check for the existence of group 2 or group 3 for the value.

([^\s:]+):(?:\[([^][]+)]|(\S+))

The pattern matches:

([^\s:]+) Capture group 1, match any char except a whitespace char or :
: Match the :
(?: Non capture group
- \[([^][]+)] Match [ capture in group 2 any char except [ and ] and match the closing ]
- | or
- (\S+) Capture 1+ non whitespace chars in group 3
) Close non capture group

Regex demo

If an conditional is supported, you could check if group 2 has captured a [. If it did, you can capture any char except the brackets in group 3.

The values you want are then in group 1 and group 3.

([^\s:]+):(?:(\[)(?=[^][]*]))?((?(2)[^][]+|\S+))\]?

Regex demo