Regular expressions simplify pattern-matching code - 10

Quantifiers

Quantifiers are probably the most confusing regex constructs to understand. Part of that confusion comes from trying to grasp Pattern's 18 quantifier categories (organized as three major categories of six fundamental quantifier categories). Another part of that confusion comes from trying to decipher the concept of zero-length matches. Once you understand that concept and those 18 categories, much (if not all) of the confusion disappears.

Note

For brevity, this section discusses only the basics of the 18 quantifier categories and the zero-length match concept. Study The Java Tutorial's "Quantifiers" section for a more detailed discussion and more examples.

A quantifier is a regex construct that implicitly or explicitly binds a numeric value to a pattern. That numeric value determines how many times to match a pattern. Pattern's six fundamental quantifiers match a pattern once or not at all, zero or more times, one or more times, an exact number of times, at least x times, and at least x times but no more than y times.
The six fundamental quantifier categories replicate in each of three major categories: greedy, reluctant, and possessive. Greedy quantifiers attempt to find the longest match. In contrast, reluctant quantifiers attempt to find the shortest match. Possessive quantifiers also try to find the longest match. However, they differ from greedy quantifies in how they work. Although greedy and possessive quantifiers force a matcher to read in the entire text prior to attempting a first match, greedy quantifiers often cause a matcher to make multiple attempts to find a match, whereas possessive quantifiers cause a matcher to attempt a match only once.