Regular Expressions in .NET

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Introduction

Regular expressions have been widely popular in languages such as PERL and AWK and have been utilized for pattern matching, text manipulation and text searching. These languages are specifically is known for its advanced pattern matching features. Dot Net regular expressions are based on that of Perl and are compatible with Perl 5 regular expressions.

To begin with, they are not as complex as they look, especially if you start experimenting with them. I would recommend that you download a tool such as Expresso (http://www.ultrapico.com/), to become familiar with regular expressions.

Regular Expression Elements

Some of the commonly used regular expression elements are:

The following are matching substitutions:

Regular expressions could also be used to find repeating patterns by making use of backreferencing, using which you can name a pattern found and then use that reference elsewhere in expression. This naming of patterns is also useful in case we need to parse a string like free form date or time strings.

Some Example Regular Expressions

• Match a word - \btest\b
• Match all 6 letter words - \b\w{6}\b
• Match all 6 digit numbers - \b\d{6}\b
• Match any number \b\d+\b

Instead of giving loads of examples here, I suggest that you download Expresso and check its analyzer view for detailed analysis of the regular expression.

Regular Expressions in .Net

As already discussed, .Net regular expressions are based on that of Perl and are compatible with Perl 5 regular expressions. Dotnet contains a set of powerful classes that makes it even easier to use regular expressions. The classes are available in the System.Text.RegularExpressions namespace. The following is a list of classes in the namespace:

How to validate an input string in .Net

• Create a Regex object �RegexObj�
• Call RegexObj.IsMatch (subjectString ), which will return a Boolean showing validity of input string

How to perform regular expression substitution (search and replace) in .Net

• Create a Regex object �RegexObj�
• Call RegexObj.Replace ( subjectString, replaceString ), which will return a Boolean showing validity of input string

How to parse an input string in .Net

• Create a Regex object �RegexObj�, make sure to name the expressions
• Call RegexObj.Match ( subjectString ), which will return a list of matches in the input string as per the match regular expression
• Iterate through the matches to perform post parsing

Free form time parsing function in DotNet

The following is a utility function that can parse a free format time string. This could be extended to a combined date and time parser along with many more enhancements. If anyone needs further help, feel free to contact me.

private const string TIME_STR = @"^\s?(?" 
           + @"(?\d{1,2})" + @"(:(?\d{1,2}))?"
           + @"\s?((?(am|pm)))?"
           + @")\s?$";

static DateTime ParseTime (string strTime)
{
    DateTime currTime = DateTime.Now;
    DateTime finalTime = DateTime.Today;
    Match m;
    int hour = 0,
    min = 0;
    Regex regExTime = new Regex (TIME_STR, 
        RegexOptions.IgnoreCase
        | RegexOptions.CultureInvariant
        | RegexOptions.IgnorePatternWhitespace
        | RegexOptions.Compiled);

    m = regExTime.Match (strTime);

    if (m.Success)
    {

        if (m.Groups["hour"].Success)
            hour = Int32.Parse (m.Groups["hour"].Value);

        if (m.Groups["min"].Success)
            min = Int32.Parse (m.Groups["min"].Value);

        if (m.Groups["am_pm"].Success) 
            hour = ConvertAmPm (m.Groups["am_pm"].Value, hour);
    } 
    else
        throw new FormatException ("Invalid time format");

    if (hour > 23 || min > 59)
        throw new FormatException ("Invalid time format");

    finalTime = new DateTime (currTime.Year, currTime.Month,
    currTime.Day, hour, min, 0);
    return finalTime;
}

private static int ConvertAmPm (string amPm, int hour)
{
    int retHour = hour;
    amPm = amPm.ToLower();

    if (amPm.Equals("am")) 
    // all hours remain the same except the 12:00 am 

    // (which is 0000 hours)


        if (hour == 12)
            retHour = 00;
        else if (amPm.Equals("pm")) 
        // add 12 to hours except if 12:00 pm


    if (hour != 12)
        retHour = hour + 12;
    else
        throw new FormatException ("Invalid amPm flag format");
    return retHour;
}

Expresso Analysis of the regular expression used above is shown in the figure below. This should help you understand the details.

References and Further Reading

• Learn how to write a Regular Expression
• Ultra Pico � Expresso
• The 30 minute Regex Tutorial
• Regular Expression Cheat Sheet
• .Net Framework Regular Expressions (MSDN Library) (Online)