preg匹配错误的结果

Question

I have this string:

a[0]=a[27%a.length];

and this pattern

([a-z])+\[(\S)+\]\=([a-z])+\[+(.*?)+\%+([a-z])+\.length

Preg match array is this one:

array(6) {
  [0]=>
  string(18) "a[0]=a[27%a.length"
  [1]=>
  string(1) "a"
  [2]=>
  string(1) "0"
  [3]=>
  string(1) "a"
  [4]=>
  string(0) ""
  [5]=>
  string(1) "a"
}

Why is the element 4 empty instead of holding the 27?

Answer 1

I think there's one "+" too many. This regex seems to work as you would expect:

([a-z])+\[(\S)+\]\=([a-z])+\[+(.*?)\%+([a-z])+\.length

The following is even cleaner, and it matches the input string:

([a-z])+\[(\S)+\]\=([a-z])+\[(.*?)\%([a-z])+\.length

The difference is that I removed the unnecessary "+" after the squared brackets ("+" means "one or more", while there should be only one bracket in each position).

For an explanation, please refer to regex101, which has a nice formatting for the various parts of the expression: http://regex101.com/r/iB6nH1