Visual Basic.NET How to Program (PDF)

Chapter 15 Strings, Characters and Regular Expressions 677
Fig. 15.23 Regex methods Replace and Split (part 3 of 3).
Method Replace replaces text in a String with new text wherever the original
String matches a regular expression. We present two versions of this method in
Fig. 15.23. The first version (line 21) is Shared and takes three parameters—the String
to modify, the String containing the regular expression to match and the replacement
String. Here, Replace replaces every instance of "*" in testString1 with "^".
Notice that the regular expression ("\*") precedes character * with a backslash, \. Normally,
* is a quantifier indicating that a regular expression should match any number of
occurrences of a preceding pattern. However, in line 21, we want to find all occurrences of
the literal character *; to do this, we must escape character * with character \. By escaping
a special regular expression character with a \, we inform the regular-expression matching
engine to find the actual character, as opposed to what it represents in a regular expression.
The second version of method Replace (line 26) is an instance method that uses the regular
expression passed to the constructor for testRegex1 (line 14) to perform the
replacement operation. In this case, every match for the regular expression "stars" in
testString1 is replaced with "carets".
Line 15 instantiates testRegex2 with argument "\d". The call to instance method
Replace in line 40 takes three arguments—a String to modify, a String containing
the replacement text and an Integer specifying the number of replacements to make. In
other words, this version of Replace replaces the first three instances of a digit ("\d")
in testString2 with the text "digit" (line 40).
Method Split divides a String into several substrings. The original String is
broken in any location that matches a specified regular expression. Method Split returns
an array containing the substrings between matches for the regular expression. In line 45,
we use the Shared version of method Split to separate a String of comma-separated
integers. The first argument is the String to split, and the second argument is the regular
expression. In this case, we use the regular expression ",\s*" to separate the substrings
wherever a comma occurs. By matching any whitespace characters, we eliminate extra
spaces from the resulting substrings.
SUMMARY
Characters are the fundamental building blocks of Visual Basic program code. Every program is
composed of a sequence of characters that is interpreted by the compiler as a series of instructions
used to accomplish a task.