使用preg_match在php中使用正则表达式

Question

I am trying to make a php file so it can accept word for a regex (a+b)(ab+ba)* i am using preg_match and so far ive come up with this

    $subject = "a+b";
    $pattern = '(([a]{1}\+[b]{1})?([ab]{1}\+[ba]{1}))';
    preg_match($pattern, $subject, $matches);
    print_r($matches);

I am not sure if i completely understand how it works but its been few hours now I am trying to figure out.. how do i make it so it fullfills my condition

I want to match (a+b)(ab+ba)* where the first bracket (a+b) is required and the * on the second bracket (ab+ba) means that there could be zero or multiple instances of it. so it should work like this

   $subject= "(a+b)"
   Match


   $subject= "(a+b)(ab+ba)"
   Match


   $subject= "(a+b)(ab+ba)(ab+ba)"
   Match

   $subject= "(ab+ba)"
   No Match

   $subject= ""
   No Match

Answer 1

In (([a]{1}\\+[b]{1})?([ab]{1}\\+[ba]{1}))

[a]{1} (1 character in a character class ) can be written as a

[b]{1} can be written as b

[ab] (2 characters in a character class) means a or b

I think you have to escape the opening parenthesis \\( , or else you would start a capturing group.

If I am not mistaken, this might match what you are looking for:

^\\(a\\+b\\)(?:\\(ab\\+ba\\))*$

The second part is in a non capturing group (?:\\(ab\\+ba\\))* and repeated zero or more times.

Php test ouput