ABAB中的正则表达式匹配编号（必须相同）

Question

i've search many answer to match ABAB pattern that match arbitrary character sort of 1212 or 2323, have read this too but i found that the pattern to match ABAB pattern not work, it also match 4444

i try to figure out the pattern to match such subject below:

2323
4545
9898

but may not match 4444 or 5555 because that is not in ABAB pattern, i name it AAAA pattern
can someone give me clue
thanks

Answer 1

You can use this pattern:

(\d)(?!\1)(\d)\1\2

• (\d) - capture the first digit to group $1.
• (?!\1) - check that the second digit is not the first digit.
• (\d) - capture the second digit to group $2.
• \1 - match the third digit if it is the same as the first.
• \2 - match the fourth digit if it is the same as the second.

Working example: http://www.regex101.com/r/aV2uG1

This is a relatively confusing regular expression, and the task can be easily solved by a few lines of code. I'm not big on PHP, but this seems to work:

$s = '1112';
$valid = (strlen($s) === 4) && ($s[0] === $s[2]) && ($s[1] === $s[3]) &&
         ($s[0] !== $s[1]);