Repeating Character Classes
If you repeat a character class by using the ?, * or + operators, you're repeating the entire character class. You're not repeating just the character that it matched. The regex [0-9]+ can match 837 as well as 222.
If you want to repeat the matched character, rather than the class, you need to use backreferences. ([0-9])\1+matches 222 but not 837. When applied to the string 833337, it matches 3333 in the middle of this string. If you do not want that, you need to use lookaround.