Question

I want to retrieve numbers from string whose character start from 8 OR 9 OR 6 and is followed by 8 digits. 

E.G

sting string string  85689252 91585555 65987582  sting 12345678 string

Output should be:

85689252 
91585555 
65987582

Answer 1

You probably want this regex:

\b[896]\d{7}\b

I assume that the numbers are at least separated by some character other than alphanumeric and _, so I used \b to match word boundary.

If you want to relax the condition, the regex engine must support look ahead and look behind:

(?<!\d)[896]\d{7}(?!\d)

In this case, I just make sure the whole number is not preceded or followed by any digit.

Your question says followed by 8 digits, but it seems you mean the whole number should have 8 digits – so I changed my answer according to your sample input.

Use Regex.Matches to get all the string that matches the regex.