Question

I’m trying to create a regex expression to match any string that has exactly 9 digits. the digits can exist anywhere in the string.

For example, if you passed in the following strings, you’d get a match:
123456789
123aeiou456abc789

These strings would fail to provide a match
12345678
1234567890

Answer 1

Try this:

@"^(\D*\d){9}\D*$"

Or use this improved version. It assumes that you only want to match 0-9 and not other characters that represent digits in other languages. It also uses a non-capturing group to improve performance:

"^(?:[^0-9]*[0-9]){9}[^0-9]*$"

Here’s a breakdown of what it means:

^        Start of string.
(?:      Start a non-capturing group.
[^0-9]*  Match zero or more non-digits.
[0-9]    Match exactly one digit.
)        Close the group.
{9}      Repeat the group exactly 9 times.
[^0-9]*  Match zero or more non-digits.
$        End of string.

Here’s a testbed for it:

string regex = "^(?:[^0-9]*[0-9]){9}[^0-9]*$"
string[] tests = {
                     "123456789",
                     "123aeiou456abc789",
                     "12345678",
                     "1234567890"
                 };
foreach (string test in tests)
{
    bool isMatch = Regex.IsMatch(test, regex); 
    Console.WriteLine("{0}: {1}", test, isMatch);
}

Results:

123456789: True
123aeiou456abc789: True
12345678: False
1234567890: False