Regex for Alternating Numbers

I am trying to write a regex pattern for phone numbers consisting of 9 fixed digits.

I want to identify numbers that have two numbers alternating for four times such as 5XYXYXYXY

I used the below sample

number = 561616161

I tried the below pattern but it is not accurate

^5(d)(?=d1).+

can someone point out what i am doing wrong?

Answer

I would use:

^(?=d{9}$)d*(d)(d)(?:12){3}d*$

Demo

Here is an explanation of the pattern:

• ^ from the start of the number
• (?=d{9}$) assert exactly 9 digits
• d* match optional leading digits
• (d) capture a digit in 1
• (d) capture another digit in 2
• (?:12){3} match the XY combination 3 more times
• d* more optional digits
• $ end of the number