You can also use
Regex[
^]. See:
string[] digits = {"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "specific"};
string pattern = @"^\w{1}i";
Regex r = new Regex(pattern);
var result = digits.Where(word => r.IsMatch(word)).ToList();
^\w{1}i
pattern means:
^
- asserts position at start of a line
\w{1}
- matches any word character (equivalent to [a-zA-Z0-9_]), exactly one occurence
i
- find exact match, a letter i
More at:
regex101: build, test, and debug regex[
^]