1. Strings, Characters and Regular Expressions16
Strings, Characters and Regular Expressions

2. The chief defect of Henry King Was chewing little bits of string.
Hilaire Belloc
Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences.
William Strunk, Jr.
I have made this letter longer than usual, because I lack the time to make it short.
Blaise Pascal

3. The difference between the almost-right word & the right word is really a large matter—it’s the difference between the lightning bug and the lightning.
Mark Twain
Mum’s the word.
Miguel de Cervantes, Don Quixote de la Mancha

4. OBJECTIVES In this chapter you will learn:
To create and manipulate immutable character string objects of class string.
To create and manipulate mutable character string objects of class StringBuilder.
To manipulate character objects of struct Char.
To use regular expressions in conjunction with classes Regex and Match.

5. 16.1   Introduction
16.2   Fundamentals of Characters and Strings
16.3   string Constructors
16.4   string Indexer, Length Property and CopyTo Method
16.5   Comparing strings
16.6   Locating Characters and Substrings in strings
16.7   Extracting Substrings from strings
16.8   Concatenating strings
16.9   Miscellaneous string Methods
16.10   Class StringBuilder
16.11   Length and Capacity Properties, EnsureCapacity Method and Indexer of Class StringBuilder
16.12  Append and AppendFormat Methods of Class StringBuilder

6. 16.13 Insert, Remove and Replace Methods of Class StringBuilder
16.14 Char Methods
16.15 Card Shuffling and Dealing Simulation
16.16  Regular Expressions and Class Regex
Regular Expression Example
  Validating User Input with Regular Expressions
Regex methods Replace and Split
16.17 Wrap-Up

7. 16.1 Introduction FCL’s string and character processing capabilities
string’s constructors and methods
StringBuilder from System.Text namespace
Builds strings dynamically
Regex and Match from the System.Text.RegularExpressions namespace
Manipulate by patterns

8. 16.2 Fundamentals of Characters and Strings
“Building blocks” of C# source programs
Character constant
A character that is represented as an integer value
Unicode character set
International character set
String
Immutable
Series of characters treated as single unit
May include letters, digits, special characters
String literals
Sequence of characters
From System namespace
To interpret all characters literally before the beginning quotation mark

9. Performance Tip 16.1
If there are multiple occurrences of the same string literal object in an application, a single copy of the string literal object will be referenced from each location in the program that uses that string literal. It is possible to share the object in this manner, because string literal objects are implicitly constant. Such sharing conserves memory.

10. 16.3 string Constructors string Constructors
Can initialize string as if it was a primitive type
Ex: string example = “I see…”;
Can initialize string in the same way as a normal class (Eight constructors)
Ex: string example = new string( “I see…” );

11. Outline
StringConstructor .cs
(1 of 2)
Assigns string literal to string reference originalString
Set string1 to have the same string literal as originalString
One-argument constructor creates a string that contains a copy of the characters in the array argument
Three-argument constructor creates a string that contains a copy of partial characters in the array argument
Two-argument constructor creates a string that contains the character argument repeated a specified numbers of time

12. Outline
StringConstructor .cs
(2 of 2)

13. Software Engineering Observation 16.1
In most cases, it is not necessary to make a copy of an existing string. All strings are immutable—their character contents cannot be changed after they are created. Also, if there are one or more references to a string (or any object for that matter), the object cannot be reclaimed by the garbage collector.

14. 16.4 string Indexer, Length Property and CopyTo Method
Facilitates the retrieval of any character in the string
Treats a string as an array of chars
Return the character at the specific position in the string
Length property
Returns the length of the string
CopyTo Method
Copies a specified number of characters into a char array

15. Outline (1 of 2) Determine the number of characters in string1
StringMethods.cs
(1 of 2)
Determine the number of characters in string1
Print the corresponding character at the indexer position of string1

16. Outline Copy the characters from a string into a char array (2 of 2)
StringMethods.cs
(2 of 2)
Determine the number of characters in the array
Print the corresponding character at the indexer position of the array

17. Common Programming Error 16.1
Attempting to access a character that is outside a string’s bounds (i.e., an index less than 0 or an index greater than or equal to the string’s length) results in an IndexOutOfRangeException.

18. 16.5 Comparing strings Comparing String objects Method Equals or ==
Determine if the strings are the same
Returns bool value
Uses a lexicographical comparison
The integer Unicode value that represent each character in each string are compared
Method CompareTo
Returns 0 if strings are equal
Returns negative value if the string invoked is less than the string that is passed in
Returns positive value if the string invoked is greater than the string that is passed in
Method StartsWith
Determines if string instance starts with the string text passed to it as an argument
Method EndWith
Determines if string instance ends with the string text passed to it as an argument

19. Outline
StringCompare.cs
(1 of 3)
Method Equals tests two strings for equality using lexicographical comparison

20. Method CompareTo compares string objects
Operator == also tests two strings for equality using lexicographical comparison
Outline
StringCompare.cs
(2 of 3)
static method Equals tests two strings for equality using lexicographical comparison
Method CompareTo compares string objects

21. Outline
StringCompare.cs
(3 of 3)

22. Outline
StringStartEnd.cs
(1 of 2)
Method StartsWith determines if strings starts with specified characters

23. Method EndsWith determines if strings ends with specified characters
Outline
Method EndsWith determines if strings ends with specified characters
StringStartEnd.cs
(2 of 2)

24. 16.6 Locating Characters and Substrings in strings
Search for characters in string
Method IndexOf
Returns the index of first occurrence of a character or substring; -1 if not found
Method IndexOfAny
Same as IndexOf excepts it takes in an array of characters and returns the index of the first occurrence of any of the characters in the array
Method LastIndexOf
Returns the index of last occurrence of a character or substring; -1 if not found
Method LastIndexOfAny
Same as LastIndexOf excepts it takes in an array of characters and returns the index of the last occurrence of any of the characters in the array

25. Outline
StringIndexMethods .cs
(1 of 4)
Method IndexOf finds the first occurrence of character in letters
Method LastIndexOf finds the last occurrence of character in letters

26. Outline
Method IndexOf finds the first occurrence of substring in letters
StringIndexMethods .cs
(2 of 4)
Method LastIndexOf the finds last occurrence of substring in letters
Method IndexOfAny returns the index of the first occurrence of any of the characters in the array

27. Outline
Method LastIndexOfAny returns the index of the last occurrence of any of the characters in the array
StringIndexMethods .cs
(3 of 4)

28. Outline
StringIndexMethods .cs
(4 of 4)

29. Common Programming Error 16.2
In the overloaded methods LastIndexOf and LastIndexOfAny that take three parameters, the second argument must be greater than or equal to the third. This might seem counterintuitive, but remember that the search moves from the end of the string toward the start of the string.

30. 16.7 Extracting Substrings from strings
Method Substring
Creates and returns a new string by copying part of an existing string

31. Outline Beginning at index 20, copy all the characters from letters
SubString.cs
Beginning at index 20, copy all the characters from letters
Extract the characters from index 0 to 6 from letters

32. 16.8 Concatenating Strings
Method Concat or +
Returns a new string containing the combined characters from both original strings

33. Outline Concatenate string2 to string1
SubConcatenation .cs
Concatenate string2 to string1
However, string1 is not modified by method Concat

34. 16.9 Miscellaneous string Methods
Method Replace
Returns a new string replacing every occurrence of the specified phrase with another phrase in the string
Method ToLower
Returns a new lower cased version of the string
Method ToUpper
Returns a new upper cased version of the string
Method Trim
Remove all white space characters from the string

35. Outline
StringMethods2.cs
(1 of 2)
Use method Replace to return a copy of string1 in which every occurrence of ‘e’ is replaced with ‘E’
Use method ToUpper to return a copy of string1 in which every character is uppercase
Use method ToLower to return a copy of string2 in which every character is lowercase

36. Use method Trim to return a copy of string3 in which whitespace is eliminated
Outline
StringMethods2.cs
(2 of 2)

37. 16.10 Class StringBuilder Class StringBuilder
Used to create and manipulate dynamic string information
Every StringBuilder can store the number of characters specified by its capacity
Exceeding the capacity of a StringBuilder makes the capacity expand to accommodate the additional characters
The default initial capacity is 16 characters
6 overloaded constructors

38. Performance Tip 16.2
Objects of class string are immutable (i.e., constant strings), whereas object of class StringBuilder are mutable. C# can perform certain optimizations involving strings (such as the sharing of one string among multiple references), because it knows these objects will not change.

39. Namespace for class StringBuilder
Outline
Namespace for class StringBuilder
No-argument constructor creates empty StringBuilder with capacity of 16 characters
StringBuilderConst ructor.cs
One-argument constructor creates empty StringBuilder with capacity of specified (10) characters
One-argument constructor creates StringBuilder with string “hello” and capacity of 16 characters

40. 16.11 Length and Capacity Properties, EnsureCapacity Method and Indexer of Class StringBuilder
Length Property
Return number of characters currently in the StringBuilder
Capacity Property
Return number of characters that the StringBuilder can store without allocating more memory
EnsureCapacity Method
Reduce the number of times the StringBuilder’s capacity can be increased
Indexers is like that of string

41. Outline Create a new StringBuilder (1 of 2)
StringBuilderFeatu res.cs
(1 of 2)
Property Length returns the number of characters currently in the buffer
Property Capacity returns the number of characters that buffer can store without allocating more memory
Use method EnsureCapacity to set capacity to 75
Use property Length to set length to 10

42. Print corresponding character to the indexer’s position in buffer
Property Length returns the number of characters currently in the buffer
Outline
Print corresponding character to the indexer’s position in buffer
StringBuilderFeatu res.cs
(2 of 2)

43. Common Programming Error 16.3
Assigning null to a string reference can lead to logic errors if you attempt to compare null to an empty string. The keyword null is a value that represents a null reference (i.e., a reference that does not refer to an object), not an empty string (which is a string object that is of length 0 and contains no characters).

44. 16.12 Append and AppendFormat Methods of Class StringBuilder
Method Append
Appends the string representation to the end the StringBuilder
Method AppendFormat
Converts a string to a specified format, then appends it to the StringBuilder

45. Outline
StringBuilderAppen d.cs
(1 of 2)

46. Outline Append string “hello” to StringBuilder
Append a space character to StringBuilder
Append string “good bye”
StringBuilderAppen d.cs
(2 of 2)
Append “a b c d e f”
Append “a b c”
Append boolean, char, int, long, float and double
Print out results

47. Outline (1 of 2) String literal that contains formatting information
StringBuilderAppen dFormat.cs
(1 of 2)
String literal that contains formatting information
string1’s arguments
Combine the string literal and the arguments together

48. Outline Another string literal that contains formatting information
StringBuilderAppen dFormat.cs
(2 of 2)
Combine the string literal and the argument together

49. 16.13 Insert, Remove and Replace Methods of Class StringBuilder
Method Insert
Allow various types of data to be inserted at any position
Method Remove
Delete any portion of StringBuilder
Method Replace
Searches for a specified string or character and substitutes another string or character in its place

50. Outline
StringBuilderInser tRemove.cs
(1 of 3)

51. Outline
StringBuilderInser tRemove.cs
(2 of 3)
Use method Insert to insert data in beginning of StringBuilder
Use method Remove to remove character from index 10 in StringBuilder
Remove characters from indices 4 through 7

52. Outline
StringBuilderInser tRemove.cs
(3 of 3)

53. Outline (1 of 2) Replace “Jane” with “Greg” in builder1
StringBuilderRepla ce.cs
(1 of 2)
Replace “Jane” with “Greg” in builder1
Replace “g” with “G” in the first 5 characters of builder2

54. Outline
StringBuilderRepla ce.cs
(2 of 2)

55. 16.14 Char Methods Char Simple types (including char) are structs
Represents value types
structs derive from ValueType
Most methods are static
IsDigit
IsLetter
IsLetterOrDigit
IsLower
IsUpper
ToLower
ToUpper
IsPunctuation
IsSymbol

56. Convert the user’s input to a char
Outline
StaticCharMethods. cs
(1 of 3)
Convert the user’s input to a char

57. Outline Determine whether character is a digit
Determine whether character is a letter
Determine whether character is a letter or a digit
Determine whether character is a letter or a digit
StaticCharMethods. cs
(2 of 3)
Determine whether character is lowercase and uppercase, respectively
Convert character to its uppercase and lowercase, respectively
Determine whether character is a punctuation
Determine whether character is a symbol

58. (a)
Outline
(b)
(c)
StaticCharMethods. cs
(3 of 3)
(d)
(e)

59. 16.15 Card Shuffling and Dealing Simulation
This example shows how strings could be used in programs

60. Fields that represents a card
Outline
Fields that represents a card
Card.cs

61. Outline An array of Cards to represent a deck of cards (1 of 5)
DeckForm.cs
(1 of 5)
An array of strings to represent the many faces of a card
An array of strings to represent the many suits of a card

62. Outline Assign a face and a suit to every card of the deck (2 of 5)
DeckForm.cs
(2 of 5)
Store the dealt card
Display the dealt card
Notify user that no cards remain

63. Outline Create a Random object to make shuffle random (3 of 5)
DeckForm.cs
(3 of 5)
Swap cards for shuffling

64. Outline
DeckForm.cs
(4 of 5)
Shuffle cards

65. (a)
Outline
DeckForm.cs
(5 of 5)
(b)
(c)
(d)

66. 16.16 Regular Expressions and Class Regex
Regex Class
From namespace System.Text.RegularExpressions
Represents an immutable regular expression
Specially formatted strings
Method Match
Returns an object of class Match that represents a single regular expression match
Method Matches
Finds all matches of a regular expression in a string and returns an object of the class MatchCollection object containing all the Matches

67. Fig. 16.18 | Character classes.

68. 16.16.1 Regular Expression Example
The dot character “.” matches any single character except a newline character
When the dot character is followed by an asterisk, the regular expression matches any number of unspecified character except newlines
Range of characters are represented by placing a dash between two characters
Can specify that pattern should match anything other than the characters in the brackets using “^”
Ex: [^4] matches any non-digit and digits other than 4
All qualifiers are greedy
Will match as many occurrences of the pattern as possible
If qualifier is followed by a question mark, it becomes lazy
Will match as few occurrences as possible

69. Outline
Using System.Text.RegularExpressions namespace for finding patterns in the text
RegexMatches.cs
Create a Regex object with a regular expression string
string1 used for finding patterns
Find and print out all matches

70. Fig. 16.20 | Quantifiers used in regular expressions.

71. 16.16.2 Validating User Input with Regular Expressions
Match Property Success
Indicates whether there was a match
"|" matches the expression to its left or to its right
Parentheses can be used to group parts of a regular expression
Quantifiers may be applied to patterns enclosed in parentheses to create more complex regular expressions

72. Outline
Validate.cs
(1 of 7)

73. Outline
Validate.cs
(2 of 7)
The Success property indicates whether the first argument matches the pattern from the second argument

74. Outline
Validate.cs
(3 of 7)
The Success property indicates whether the first argument matches the pattern from the second argument

75. Outline
Validate.cs
(4 of 7)
The Success property indicates whether the first argument matches the pattern from the second argument

76. Outline
Validate.cs
(5 of 7)
The Success property indicates whether the first argument matches the pattern from the second argument
Hides the form
Terminate program

77. (a)
Outline
Validate.cs
(6 of 7)
(b)

78. (c)
Outline
Validate.cs
(7 of 7)
(d)

79. 16.16.3 Regex Methods Replace and Split
Method Replace
Replaces text in a string with new text wherever the original string matches a regular expression
Method Split
Divides a string into several substrings
The original string is broken at delimiters that match a specified regular expression
Returns an array containing the substrings

80. Outline (1 of 2) strings used for finding patterns
RegexSubstitution. cs
(1 of 2)
strings used for finding patterns
Create a Regex object
Replaces “*” with “^” in testString1
Replaces “stars” with “carets” in testString1
Replaces every word with “word”
Replaces the first 3 digits with “digit”

81. Outline
Split the string at commas and have each substring as an element of a string array
RegexSubstitution. cs
(2 of 2)