Strange result of using asterisk * quantifier

问题

I am trying to practice asterisk * quantifier on a simple string, but while i have only two letters, the result contains a third match.

<?php
$x = 'ab';

preg_match_all("/a*/",$x,$m);
echo '<pre>';
var_dump($m);
echo '</pre>';
?>

the result came out:

 array(1) {
  [0]=>
    array(3) {
     [0]=> string(1) "a"
     [1]=> string(0) ""
     [2]=> string(0) ""
    }
 }

As i understand it first matched a then nothing matched when b, so the result should be

  array(1) {
  [0]=>
    array(2) {
     [0]=> string(1) "a"
     [1]=> string(0) ""
    }
 }

So what is the third match?

回答1:

From using a regex demo tool here, we can see that the first match is a, while the second and third matches are the zero width delimiters in between a and b, and also in between b and the end of the string.

Keep in mind that the behavior of preg_match_all is to repeatedly take the pattern a* and try to apply it sequentially to the entire input string.

I suspect that what you really want to use here is a+. If you examine this second demo, you will see that with a+ we only get a single match, for the single a letter in ab. So, I vote for using a+ here to resolve your problem.

回答2:

Your regular expression '/a/*' Matches zero(empty) or more consecutive a characters.

Example : if you try to match '/a*/' to an empty string it will return one match because * refer to nothing or more . see here

the preg_match_all continues to look until finishning processing the entire string. Once match is found, it remainds of the string to try and apply another match.

来源：https://stackoverflow.com/questions/54419455/strange-result-of-using-asterisk-quantifier

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php

regex

pcre