Question

I am a newbie at regular expression.
I have strings with formats like
1) 3.72 million people (country rank: 6th) (2004 estimate)
2) 10000 people (2007 estimate)

I would like extract the population number and the time from these two kinds of string. How can I do it in a regular expression in C#. Or do I need to write multiple regular expressions?

Answer 1

Here is a starting point:

(?<population>\d\(.\d+)?)  #capturing group named "population"
                           #that is a number, optionally followed by a
                           #decimal point and at least one number
\s*                        #followed by one or more spaces
(?<magnitude>thousand|(m|b)illion)? #optional capturing group named "magnitude"
                                    # that matches "thousand", "million", or "billion"
\s*                        #one or more whitespace characters
people                     #the literal "people"
.*                         #match any number of characters
\(                         #Find literal opening parentheses...
   (?<year>\d{4})          #...followed by a four-digit year...
\s                         #...followed by a space...
estimate\)                 #...followed by the phrase "estimate)"
\s*$                       #followed by optional whitespace
                           #and the end of the string

A simple driver to show usage:

class Program
{
/// Generate test strings
static IEnumerable<string> Generator()
{
    yield return "3.72 million people (country rank: 6th) (2004 estimate)";
    yield return "10000 people (2007 estimate)";
}

public static void Main()
{
    string expression = @"
(?<population>\d(.\d+)?)  #capturing group named 'population'
                           #that is a number, optionally followed by a
                           #decimal point and at least one number
\s*                        #followed by one or more spaces
(?<magnitude>thousand|(m|b)illion)? #optional capturing group named 'magnitude'
                                    # that matches 'thousand', 'million', or 'billion'
\s*                        #one or more whitespace characters
people                     #the literal 'people'
.*                         #match any number of characters
\(                         #Find literal opening parentheses...
   (?<year>\d{4})          #...followed by a four-digit year...
\s                         #...followed by a space...
estimate\)                 #...followed by the phrase 'estimate'
\s*$                       #followed by optional whitespace
                           #and the end of the string";

    RegexOptions options = 
        RegexOptions.IgnorePatternWhitespace // allow whitespace/comments
        | RegexOptions.IgnoreCase
        | RegexOptions.ExplicitCapture; // Only capture named groups

    Regex r = new Regex(expression, options);
    foreach (var test in Generator())
    {
        Match match = r.Match(test);
        if (!match.Success)
            Console.WriteLine("Could not match {0}", test);
        else
        {
            double population = double.Parse(match.Groups["population"].Value);
            if (match.Groups["magnitude"].Success) // magnitude is optional
                                                   // but if present, need to
                                                   // multiply population
            {
                switch (match.Groups["magnitude"].Value.ToLower())
                {
                    case "thousand": population *= 1000; break;
                    case "million": population *= 1E6; break;
                    case "billion": population *= 1E9; break;
                    default: throw new FormatException("Unexpected value in magnitude group");
                }
            }
            int year = int.Parse(match.Groups["year"].Value);
            Console.WriteLine("In {0}, population was {1} people.", year, population);
        }
    }
}

Outputs:

In 2004, population was 3720000 people.
In 2007, population was 10000 people.