Скачать или смотреть Lesson - 2 : REGEX - Predefined Character classes and Quantifiers in REGEX Java Programming

Quantifiers allow you to specify the number of occurrences to match against. For convenience, the three sections of the Pattern API specification describing greedy, reluctant, and possessive quantifiers are presented below. At first glance it may appear that the quantifiers X?, X?? and X?+ do exactly the same thing, since they all promise to match "X, once or not at all". There are subtle implementation differences which will be explained near the end of this section.

Greedy Reluctant Possessive Meaning
X? X?? X?+ X, once or not at all
X* X*? X*+ X, zero or more times
X+ X+? X++ X, one or more times
X{n} X{n}? X{n}+ X, exactly n times
X{n,} X{n,}? X{n,}+ X, at least n times
X{n,m} X{n,m}? X{n,m}+ X, at least n but not more
than m times

Let's start our look at greedy quantifiers by creating three different regular expressions: the letter "a" followed by either ?, *, or +. Let's see what happens when these expressions are tested against an empty input string "":


Enter your regex: a?
Enter input string to search:
I found the text "" starting at index 0 and ending at index 0.

Enter your regex: a*
Enter input string to search:
I found the text "" starting at index 0 and ending at index 0.

Enter your regex: a+
Enter input string to search:
No match found.