CODEWITHSUNDEEP
javaregex
Bruce
Bruce

Reputation: 8849

Pattern matcher using Greedy and Reluctant

In java regex I have read about Greedy and Reluctant Quantifiers. They mentioned as

A reluctant or "non-greedy" quantifier first matches as little as possible. So the .* matches nothing at first, leaving the entire string unmatched

In this example

source: yyxxxyxx
pattern: .*xx

greedy quantifier * and produces

0 yyxxxyxx

reluctant qualifier *?, and we get the following:

0 yyxx
4 xyxx

Why result of yxx, yxx not possible even it is the smallest possible value?

Upvotes: 1

Views: 259

Answers (1)

Robin
Robin

Reputation: 9644

The regex engine returns the first and leftmost match it find as a result.

Basically it tries to match the pattern starting from the first character. If it doesn't find a corresponding match, the transmission jumps in and it tries again from the second character, and so on.

If you use a+?b on bab it will first try from the first b. That doesn't work, so we try from the second character.

But here it finds a match right from the first character. Starting from the second isn't even considered, we found a match so we return.

If you apply a+?b on aab, we try at the first a and find an overall match: end of story, no reason to try anything else.

To sum up: the regex engine goes from the left to the right, so laziness can only affect the right side length.

Upvotes: 2

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