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In java, what's the difference between "\\d+" and "\\d++"?
I know ++ is a possessive quantifier, but what's the difference in matching the numeric string?
What string can match "\\d+" but can't with "\\d++"?
Possessive quantifier seems to be significant with quantifier ".*" only. Is it true?
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\d (digit) matches any single digit (same as [0-9] ). The uppercase counterpart \D (non-digit) matches any single character that is not a digit (same as [^0-9] ). \s (space) matches any single whitespace (same as [ \t\n\r\f] , blank, tab, newline, carriage-return and form-feed).
Backslashes in Java. The backslash \ is an escape character in Java Strings. That means backslash has a predefined meaning in Java. You have to use double backslash \\ to define a single backslash. If you want to define \w , then you must be using \\w in your regex.
\\w+ matches all alphanumeric characters and _ . \\W+ matches all characters except alphanumeric characters and _ . They are opposite.
\\s - matches single whitespace character. \\s+ - matches sequence of one or more whitespace characters.
Possessive quantifiers will not back off, even if some backing off is required for the overall match.
So, for example, the regex \d++0 can never match any input, because \d++ will match all digits, including the 0 needed to match the last symbol of the regex.
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\d+ Means:\d means a digit (Character in the range 0-9), and + means 1 or more times. So, \d+ is 1 or more digits.
\d++ Means from Quantifiers
This is called the possessive quantifiers and they always eat the entire input string, trying once (and only once) for a match. Unlike the greedy quantifiers, possessive quantifiers never back off, even if doing so would allow the overall match to succeed.
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