Regex for 8 digit number beginning with two similar numbers?

Question

I would like to validate 8 digit phone numbers that start with a double digit greater than 2.
eg: 33452334 would validate while 32222222 would not.
This is what I tried but it's not working since it doesn't make sure the two first digits are similar:

^[3-9]{2}\d{6}$

Answer 1

Use this regex:

^([3-9])\1\d{6}$

• ( ) to capture,
• \1 to use a back reference.