Combining these two Regular Expressions into one

Question

I have the following in C#:

public static bool IsAlphaAndNumeric(string s)
{
    return Regex.IsMatch(s, @\"[a-zA-Z]+\") 
        && Regex.IsMatch(s,         


        

Answer 1

The following is not only faster than the other lookahead constructs, it is also (in my eyes) closer to the requirements:

[a-zA-Z\d]((?<=\d)[^a-zA-Z]*[a-zA-Z]|[^\d]*\d)

On my (admittedly crude test) it runs in about half the time required by the other regex solutions, and has the advantage that it will not care about newlines in the input string. (And if for some reason it should, it is obvious how to include it).

Here is how (and why) it works:

Step 1: It matches a single character (let us call it c) that is a number or a letter.
Step 2: It does a lookbehind to check if c is a number. If so:
Step 2.1: It allows an unlimited number of characters that are not a letter, followed by a single letter. If this matches, we have a number (c) followed by a letter.
Step 2.2: If c is not a number, it must be a letter (otherwise it would not have been matched). In this case we allow an unlimited number of non-digits, followed by a single digit. This would mean we have a letter (c) followed by a number.