Difference between regex quantifiers plus and star

Question

I try to extract the error number from strings like \"Wrong parameters - Error 1356\":

 Pattern p = Pattern.compile(\"(\\\\d*)\");
 Matcher m =          


        

Answer 1

With the pattern /d+ at least one digit will need to be reached, and then the match will return all subsequent characters until a non-digit character is reached.

/d* will match all the empty strings (zero or more), as well at the match. The .Net Regex parser will return all these empty string groups in its set of matches.

Answer 2

\d* Eat as many digits as possible (but none if necessary)

\d* means it matches a digit zero or more times. In your input, it matches the least possible one (ie, zero times of the digit). So it prints none.

\d+

It matches a digit one or more times. So it should find and match a digit or a digit followed by more digits.

Answer 3

The * quantifier matches zero or more occurences.

In practice, this means that

\d*

will match every possible input, including the empty string. So your regex matches at the start of the input string and returns the empty string.

Answer 4

but none if necessary means that it will not break the regex pattern if there is no match. So \d* means it will match zero or more occurrences of digits.

For eg.

\d*[a-z]*

will match

abcdef

but \d+[a-z]*

will not match

abcdef

because \d+ implies that at least one digit is required.

Answer 5

Simply:

\d* implies zero or more times

\d+ means one or more times