Regular Expressions Alternation (Pipe) trick

I had a problem with RegEx when engine matches more then one option from alternation group.

Example is like this one - I tried to match seasons like summer 2004 or spring 05. Pattern I made worked fine until I jump into problems when more then one option is listed in input string, like spring summer 2005.


First pattern: (?spring|summer|autumn|winter)(\s)?(?(?:\d{4}|\d{2}))
Input Value: spring 2005
Matches: spring 2005

First pattern: (?spring|summer|autumn|winter)(\s)?(?(?:\d{4}|\d{2}))
Input Value: spring summer 2005
Matches: summer2005

In this case I wanted spring 2005 to be matched. With listed pattern that is not possible. Since summer is listed after spring match for spring can not be found by the engine. I had to find a way to skip word summer somehow.

After several different tests I found solution. What I realized is that in input string I have 2 times following pattern (pseudo code) - season + whitespace. From that point it was easy to find make appropriate pattern.

2nd pattern: ((?spring|summer|autumn|winter)(\s))+(?(?:\d{4}|\d{2}))
Input Value: spring summer 2005
Matches: spring summer 2005.

Keep in mind that Season group has in this case two captures (for spring and summer) and in order to spring value you need to call first item in captures collection, like:

m.Groups["Season"].Captures[0].Value.

Also, captures property works fine when there is only one match.

Every time when you get unexpected results from regex engine analyze what are reasons for unexpected result and you will understand better engine that works under regular expressions in .Net.

One great reference for RegEx I use every time when I need to parse some strings:
http://www.regular-expressions.info

Bye until next tip I found :)

Enumerators in drop down.

There are very useful methods when you need to fill drop down box with values from some enumerator.

For instance, if you have enumerator like this (VB.NET code):

Public Enum FavoriteColours
Red = 1
White = 2
Yellow = 3
Green = 4
Blue = 5
End Enum

you can easily loop through enumerator keys and values and fill drop down control.

Dim enumNames As String() = System.Enum.GetNames(GetType(FavoriteColours))

Dim enumValues As System.Array = System.Enum.GetValues(GetType(FavoriteColours))

For i As Integer = 0 To enumValues.Length - 1
ctrlVehicleTypeDropDown.Items.Add(New ListItem(enumNames(i), & _
Cstr(Cint(System.Enum.ToObject(GetType(FavoriteColours))))))
Next

Generally speaking, you can fill any list type of control directly form enumerator.

Fill free to use it :)