can't understand the difference between * and + quantifiers in REGEX

Question

I am new to regex, and as i have studied, the * matches zero or more and + matches one or more, so i started to test this:

<?php
preg_match("/a/", 'bbba',$m);
preg_match("/a*/", 'bbba',$o);
preg_match("/a+/", 'bbba',$p);
echo '<pre>';
    var_dump($m);
    var_dump($o);
    var_dump($p);
echo '</pre>';  
?>

but the result is that * didn't match any thing and returned empty while the letter a exists:

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

so what i miss here.

Because `a*` also match empty, that why it shows an empty result — , Jan 28 '19 at 09:38
@catcon thank you, i have transfered to `preg_match_all` and now i get the result of every step stored in array — Mohamed Omar, Jan 28 '19 at 09:46

JGNI · Accepted Answer · 2019-01-28T09:46:55.750

4

/a/ matches the first a in bbba

/a*/ matches 0 or more a characters. There are 0 a characters between the start of the string and the first b so it matches there.

/a+/ matches 1 or more a characters so it matches the first a character

The thing to note here is that a regex will try and match as early in the string it is checking as possible.

edited Jan 28 '19 at 09:46

answered Jan 28 '19 at 09:43

JGNI

3,524
9
19

This is 'lazy' behavior. There is a good explanation for this behavior with regard to regular expressions [here][1]. [1]: https://stackoverflow.com/questions/2301285/what-do-lazy-and-greedy-mean-in-the-context-of-regular-expressions – James Sheils Jan 28 '19 at 09:46

Islam Elshobokshy · Answer 2 · 2019-01-28T09:44:55.110

2

* means that the preceding item will be matched zero or more times.

+ means that the preceding item will be matched one or more times.

Also a* match empty, that why it shows an empty result. You can use preg_match_all("/a*/", 'bbba',$o); and then filter the results on the non-empty values of the array resulting.

edited Jan 28 '19 at 09:44

answered Jan 28 '19 at 09:36

Islam Elshobokshy

8,788
6
20
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score 2 · Answer 3 · answered Jan 28 '19 at 09:46

2

a* means match string which may NOT contain a because * matches zero or more,
hence pattern a* will match even empty string. To see all matches you can use preg_match_all, like:

<?php
preg_match_all("/a*/", 'bbba', $o);
var_dump($o);

as result you will see:

array(1) {
  [0]=>
  array(5) {
    [0]=>
    string(0) ""
    [1]=>
    string(0) ""
    [2]=>
    string(0) ""
    [3]=>
    string(1) "a"
    [4]=>
    string(0) ""
  }
}

hope it will help you.

answered Jan 28 '19 at 09:46

cn007b

14,506
6
48
62

1

Thank you very much, this line made it more clear `match string which may NOT contain a`, which integrates with @JGNI's answer. thank you both – Mohamed Omar Jan 28 '19 at 09:54
@MohamedOmar welcome any time!) – cn007b Jan 28 '19 at 11:32
I think it will be more accurate if we said (matches any nothing or a). because if matched any string that may not contain, would mean to match any thing, say b. – Mohamed Omar Jan 28 '19 at 11:40
In the result you showed here, any nothing is returned and a if there is, but nothing more – Mohamed Omar Jan 28 '19 at 11:41

can't understand the difference between * and + quantifiers in REGEX

3 Answers3