How do greedy and reluctant quantifiers work

Greedy versus reluctant versus possessive quantifiers

I have never heard the exact terms "belching" or "stepping back" before. The term that would replace this is "backtracking", but "regurgitate" seems to be as good a term as any for "the content that was tentatively accepted before backtracking and thrown away".

The important thing about most regex engines is that they Backtracking: You provisionally accept a potential partial match while trying to match the entire contents of the regex. If the regex cannot be fully allocated on the first try, the regex engine backtrack at one of their matches. Attempting to repeat __, __, __, alternately, or __ differently and try again. (And yes, this process can take a long time.)

The first example uses the greedy quantifier. * To find "anything", zero or more times followed by the letters "f" "o" "o". Since the quantifier is greedy, it eats. * Part of the expression first the entire input string. At this point the total printout cannot be successful because the last three letters ("f" "o" "o") have already been used up (from whom?).

The last three letters, and have already been used up by the initial part of the rule. However, the next element in the regex,, no longer contains anything in the input string. The engine is forced to backtrack Put __ on the initial match and try to match all but the last character. (It could be that smart And is a backtrack to all-but-the-last-three as it contains three literal terms, but I am not aware of any implementation details at this level.)

So the matcher is slowly withdrawing (from right to left?) until the occurrence of "foo" on the far right has been suppressed again (what does that mean? =), where

This means that provisionally included in the search for __. Since this attempt failed, the regex engine accepts one less character in __. If there had been a successful match in this example in front the __, the engine would probably try to shorten the __ match (right to left as you showed out because it's a greedy qualifier) ​​and if it is unable to get the entire input to match, it may be forced to make the matches in front reevaluate the __ in my hypothetical example.

point, the match is successful and the search ends.

The second example is reluctant, which is why first (from whom?) "nothing" is consumed. Because "foo"

The initial nothing is consumed by __, which consumes the shortest possible amount of everything that allows the rest of the regular expression to match.

does not appear at the beginning of the string, it is forced to swallow (who swallows?) the

Again, the __ consumes the first character after tracing the original error of matching the entire regex with the closest possible match. (In this case, the regex engine extends the match for __ from left to right because __ is reluctant to do it.)

first letter (an "x") that triggers the first match on 0 and 4. Our test cable continues until the input string is exhausted. There is another match at 4 and 13.

The third example does not find a match because the quantifier is possessive. In this case, the entire input string of. * +, (how?) consumed.

A __ will consume and as much as possible is not tracedto find new matches when the regex as a whole doesn't find a match. Since the possessive form doesn't backtrack, you probably won't see many uses with __, but with character classes or similar restrictions: __.

This can dramatically speed up regex matching by telling the regex engine to never undo potential matches if something doesn't match. (If you had to handwrite all of the matching code, it would be akin to using to push back an entered character. It would be very similar to the naive code you might write on a.) First attempt: Except that regex- Engines are much better than a single push back sign, they can all rewind back to zero and try again. :)

Not only can you use it to achieve potential speedups, but you can also write regular expressions that exactly match the requirements you need to meet. I'm having trouble finding a simple example :) but writing a regex with possessive-vs-greedy quantifiers can result in different matches, and one or the other may be more appropriate.

there is nothing left at the end of the expression to satisfy the "foo". Use a possessive quantifier for situations where you want to capture everything without ever stepping back (what does resigning mean?); it will surpass

"Withdraw" in this context means "withdraw" - a preliminary partial adjustment is discarded in order to try another partial adjustment, which may or may not be successful.

the equivalent greedy quantifier in cases where the match is not found immediately.