What's the difference between (ab)+? and (ab)+ when I use it in boost:regex?

Question

+? Matches the previous atom one or more times, while consuming as little input as possible.

I don't understand this,can someone help me?

Answer 1

(ab)+? will try to match ab which occurs at least once and it will return as soon as it finds the pattern, hence it's lazy.

(ab)+ will try to match all occurrences of ab which occur at least once and then it will return, hence it's greedy.

Demo: https://regex101.com/r/rC2oB9/1 and https://regex101.com/r/hP7lM9/1

Notice that in first demo first occurrence of ab was matched (which is highlighted) whereas in second demo all occurrences of ab were matched (highlighted)

Note: A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data.

See: What do lazy and greedy mean in the context of regular expressions?

Answer 2

+ and ? is the quantifiers.

• + : Match 1 or more times,
• ? : Match 1 or 0 times,

By default, a quantified subpattern is "greedy", that is, it will match as many times as possible (given a particular starting location) while still allowing the rest of the pattern to match. If you want it to match the minimum number of times possible, follow the quantifier with a "?".
Note that the meanings don't change, just the "greediness":

+? : Match 1 or more times, not greedily.

For more info see this : Quantifiers