Visual Basic.NET How to Program (PDF)

Chapter 15 Strings, Characters and Regular Expressions 647
cally to those described above except that they search for sequences of characters (or substrings)
that are specified by their String arguments.
Lines 55–79 use methods IndexOfAny and LastIndexOfAny, which take an
array of characters as the first argument. These versions of the methods also perform identically
to those described above except that they return the index of the first occurrence of
any of the characters in the character array argument.
Common Programming Error 15.2
In the overloaded methods LastIndexOf and LastIndexOfAny that take three parameters,
the second argument must always be bigger than or equal to the third argument. This
might seem counterintuitive, but remember that the search moves from the end of the string
toward the start of the string. 15.2
15.8 Extracting Substrings from Strings
Class String provides two Substring methods, which are used to create a new
String object by copying part of an existing String object. Each method returns a new
String object. The application in Fig. 15.7 demonstrates the use of both methods.
1 ' Fig. 15.7: SubString.vb
2 ' Demonstrating the String Substring method.
3
4 Imports System.Windows.Forms
5
6 Module modSubString
7
8 Sub Main()
9 Dim letters As String = "abcdefghijklmabcdefghijklm"
10 Dim output As String
11 Dim quotes As Char = ChrW(34)
12
13 ' invoke SubString method and pass it one parameter
14 output = "Substring from index 20 to end is " & _
15 quotes & letters.Substring(20) & quotes & vbCrLf
16
17 ' invoke SubString method and pass it two parameters
18 output &= "Substring from index 0 to 6 is " & _
19 quotes & letters.Substring(0, 6) & quotes
20
21 MessageBox.Show(output, _
22 "Demonstrating String method Substring")
23 End Sub ' Main
24
25 End Module ' modSubString
Fig. 15.7 Substrings generated from Strings.