regex: question mark followed by colon as an alternative

Question

In rails cucumber there is this regex When /^(?:|I )go to (.+)$/ do |page_name|

I know ?: is a non-capturing group but what does it mean when it is there as an alternative separated by | ?

Answer 1

This isn't a special group, it just means "match nothing or I": http://www.rubular.com/r/H3iJFLXaab

This should be the same as writing (?:I )?
^{(or to be more precise, (?:I )?? - because the empty string has precedence over I, see also Is the lazy version of the 'optional' quantifier ('??') ever useful in a regular expression? )}