std::regex_match and lazy quantifier with strange behavior

Question

I know that:
Lazy quantifier matches: As Few As Possible (shortest match)

Also know that the constructor:

basic_regex( ...,
             


        

Answer 1

I don't see any inconsistency. regex_match tries to match the whole string, so s?/.+?/g? lazily expands till the whole string is covered.

These "diagrams" (for regex_search) will hopefully help to get the idea of greediness:

Non-greedy:

a.*?a: ababa
a|.*?a: a|baba
a.*?|a: a|baba  # ok, let's try .*? == "" first
# can't go further, backtracking
a.*?|a: ab|aba  # lets try .*? == "b" now
a.*?a|: aba|ba
# If the regex were a.*?a$, there would be two extra backtracking
# steps such that .*? == "bab".

Greedy:

a.*?a: ababa
a|.*a: a|baba
a.*|a: ababa|  # try .* == "baba" first
# backtrack
a.*|a: abab|a  # try .* == "bab" now
a.*a|: ababa|

And regex_match( abc ) is like regex_search( ^abc$ ) in this case.