What does [++]*+ stand for in regexp

Question

I'm using libphonenumber and I try to check some phone numbers if they are valid with regexp pattern VALID_PHONE_NUMBER which can be found here and it looks like this

private static final String VALID_PHONE_NUMBER =
      DIGITS + "{" + MIN_LENGTH_FOR_NSN + "}" + "|" +
      "[" + PLUS_CHARS + "]*+(?:[" + VALID_PUNCTUATION + STAR_SIGN + HASH_SIGN +"]*" + DIGITS + "){3,}[" +
      VALID_PUNCTUATION + STAR_SIGN + HASH_SIGN + VALID_ALPHA + DIGITS + "]*";

On my Android phone this expression is compiled to following

\p{Nd}{1}|[++]*+(?:[-x--?-?--/  ­?? ()()[].\[\]/~?~~*#]*\p{Nd}){3,}[-x--?-?--/  ­?? ()()[].\[\]/~?~~*#DEFGABCLMNOHIJKUTWVQPSRYXZdefgabclmnohijkutwvqpsryxz\p{Nd}]*

What does it mean [++]*+

Does it mean "plus-or-plus zero-or-more-times and then plus?

Does it make any sense to have two pluses next to each other?

Answer 1

[++] matches a + or a +

*+ matches the [++] zero or more times, possessively

See: http://www.regular-expressions.info/possessive.html

Answer 2

The rightmost + is fairly new. As others have said, [++] means "find a single +". with the * quantifier: [++]* means "zero or more plus signs".

The extra rightmost + in [++]*+ is not supported by all Regular Expression engines, but means "do not do any backtracking", i.e. do a "maximum match". Contrast with ?, which does a minimum match.

See also this question

Answer 3

+ within character class is treated as a literal..It should be [+]*+

*+ is Possessive Quantifier..It doesn't backtrack..

Answer 4

1. Yes
2. No

A character class ([...]) ignores duplicate characters.
Therefore, [++] is equivalent to \+.

Answer 5

The second + is redundant both inside and outside the character class.

Within the character class [++] it's just ignored. Outside the character class it acts as a quantifier (one or more) but isn't valid because except when preceded with another quantifier like * here (for zero or more) as then it denotes possessiveness.

More info at Possessive Quantifiers

You can make a quantifier possessive by placing an extra + after it. E.g. * is greedy, *? is lazy, and *+ is possessive. ++, ?+ and {n,m}+ are all possessive as well.