Double plusses in regex

I have seen several regular expressions that have two plusses in a row. What exactly does this mean? One or more of one or more of the pattern. If the pattern matches in 开发者_开发知识库the first place, why would the second match be necessary?

Examples:

[a-zA-Z0-9_]++
[^/.,;?]++

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They're called possessive quantifiers.