我怎样才能使一个正则表达式应只允许2个字符M或F

Question

I would like to valid a string if it consists of M or F . If possible i would also like to know if some one enters Male or Female i would like to get the result as M or F .. But my first case is to check whether the string has M or F . In any other character i would like to make my result as false.

Answer 1

If there are only 4 valid choices, then I reccomend you not use regex. You could use an extension method:

public static bool InList<T>(this T item, params T[] list)
{
    return list.Contains(item);
}

Then in your code you could use:

myString.InList("M", "Male", "F", "Female")

That would return TRUE if it was one of the four choices. After you validate the gender, you could just grab the first letter using .SubString().

Answer 2

Try this -

var match = Regex.Match("M", @"^M(ale)?$|^F(emale)?$");
var result = match.Success;

Answer 3

Use the regexp ^(M|Male|F|Female)$ in ignore-case mode, if it matches, get the first character and uppercase it. Precompile the regexp to get an efficient finite state machine. The result will then be machine code like this:

if s[0] == 'M':
  if s[1] == \0: return True
  if s[1] != 'a': return False
  if s[2] != 'l': return False
  if s[3] != 'e': return False
  if s[4] == \0: return True
  return False
if s[0] == 'F':
  if s[1] == \0: return True
  if s[1] != 'e': return False
  if s[2] != 'm': return False
  if s[3] != 'a': return False
  if s[4] != 'l': return False
  if s[5] != 'e': return False
  if s[6] == \0: return True
  return False
return False

Hard to beat this in the number of CPU cycles.

Answer 4

I think you are looking at this wrong.

What you need is a drop-down list or a couple of radio-buttons constraining the user's choices and returning exactly the data you want.

Otherwise, your users will have problems with your software. Some will make typos, some may type a space after the word, some of them will use a "." after M or F because it's an abbreviation. Additionally, you need to explain that you want them to write M or F. Users don't read, so anything which requires an explanation is an annoyance.

A question with one answer only out of a few possible answers should be asked with a radio button or a drop down list.

If you really want the user to write M or F, note that none of the previous answers posted allow you to get "m" or "f" as the result if the user writes "Male" or "Female". If you go that route, you need something like this:

your_string.toUpper();
if (your_string.contains("M", "MALE", "F", "FEMALE"))
{
   switch (your_string)
   {
       case "M":
          return "M";
       case "Male":
          return "M";
       case "F":
          return "F";
       case "Female":
          return "M";
       default:
          return false;
    }
}

This works, but it's much better to use the right UI tools for the job.