regex - named group only if another named group matches

A toggle command can be used as follows:

toggle
toggle n
toggle >hotspot y
toggle >hotspot
toggle @location>hotspot n
toggle @location>hotspot

my regex at the moment is the following:

^toggle(?>\s(?>@(?'location'\w+))?(?>>(?'hotspot'\w+))?)?(?>\s(?'value'n|y))?$

this one however allows the following strings to match:

toggle @location
toggle @location n

I would like to allow the named group "location" (prefixed by "@") only if the named group "hotspot" (prefixed by ">") matches.

Solution

In the pattern that you tried, the group hotspot should not be optional.

There are also a few atomic groups (?> which are not necessary if you change the grouping a bit where the whitespace char can be matched optionally.

This part n|y can also be written as a character class [ny]

^toggle(?:\s(?:@(?'location'\w+))?>(?'hotspot'\w+))?(?:\s(?'value'[ny]))?$

Explanation

^ Start of string
toggle Match literally
(?: Non capture group
- \s Match a whitespace char
- (?:@(?'location'\w+))? Optionally match @, capture in group location 1+ word chars
- >(?'hotspot'\w+) Match >, capture in group hotspot 1+ word chars
)? Close group and make it optional
(?: Non capture group
- \s(?'value'[ny]) Match a whitespace char, capture in group value either n or y
)? Close group and make it optional
$ End of string

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