Wildcard and Metacharacter Ranges Used in Regular Expressions in Visual Basic .NET

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Wildcard and Metacharacter Ranges Used in Regular Expressions
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Metacharacter *
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Description Matches the preceding character or subexpression zero or more times. For example, zo* matches z and zoo. The * character is equivalent to {0,}. Subexpressions are discussed later in this chapter. Matches the preceding character or subexpression one or more times. For example, zo+ matches zo and zoo, but not z. The + character is equivalent to {1,}. Matches the preceding character or subexpression zero or one time. For example, do(es) matches do or does. The character is equivalent to {0,1}. The n is a non-negative integer. Matches the preceding character or subexpression exactly n times. For example, o{2} does not match the o in Bob, but it does match the two o s in food. The n is a non-negative integer. Matches the preceding character or subexpression at least n times. For example, o{2,} does not match the o in Bob and does match all the o s in foooood. The sequence o{1,} is equivalent to o+. The sequence o{0,} is equivalent to o*. The m and n are non-negative integers, where n <= m. Matches the preceding character or subexpression at least n and at most m times. For example, o{1,3} matches the first three o s in fooooood, and o{0,1} is equivalent to o . Note that you cannot put a space between the comma and the numbers. When this character immediately follows any of the other quantifiers [*, +, , {n}, {n,}, {n,m}], the matching pattern is nongreedy. A nongreedy pattern matches as little of the searched string as possible, whereas the default greedy pattern matches as much of the searched string as possible. For example, in the string oooo, o+ matches a single o, whereas o+ matches all o s.
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{n,}
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{n,m}
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3
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Table 3-3
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Wildcard and Metacharacter Ranges Used in Regular Expressions
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Metacharacter . x|y [xyz] [a-z]
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Description Matches any single character except \n. To match any character including the \n, use a pattern such as [\s\S]. Matches either x or y. For example, z|food matches z or food, and (z|f)ood matches zood or food. A character set. Matches any one of the enclosed characters. For example, [abc] matches the a in plain. A range of characters. Matches any character in the specified range. For example, [a-z] matches any lowercase alphabetic character in the range a through z.
Regular expressions also provide special characters to represent common character ranges. You could use [0-9] to match any numeric digit, or you can use \d. Similarly, \D matches any non-numeric character. Use \s to match any white-space character, and use \S to match any non-white-space character. Table 3-4 summarizes these symbols.
Table 3-4
Symbols Used in Regular Expressions
Metacharacter \d \D \s \S \w \W
Description Matches a digit character. Equivalent to [0-9]. Matches a nondigit character. Equivalent to [^0-9]. Matches any white-space character, including Space, Tab, and form-feed. Equivalent to [\f\n\r\t\v]. Matches any non-white-space character. Equivalent to [^\f\n\r\t\v]. Matches any word character, including underscore. Equivalent to [A-Za-z0-9_]. Matches any nonword character. Equivalent to [^A-Za-z0-9_].
To match a group of characters, surround the characters with parentheses. For example, foo(loo){1,3}hoo would match fooloohoo and fooloolooloohoo but not foohoo or foololohoo. Similarly, foo(loo|roo)hoo would match either fooloohoo or fooroohoo. You can apply any wildcard or other special character to a group of characters.
Lesson 1: Forming Regular Expressions
You can also name a group so that later you can retrieve the data that matched the group. To name a group, use the format ( <name>pattern). For example, the regular expression foo( <mid>loo|roo)hoo would match fooloohoo. Later, you could reference the group mid to retrieve loo. If you used the same regular expression to match fooroohoo, mid would contain roo.
How to Match Using Backreferences
Backreferencing uses either named groups and the \k metacharacter or a backslash followed by a one-digit number to allow you to search for other instances of characters that match a wildcard. Backreferences provide a convenient way to find repeating groups of characters. They can be thought of as a shorthand instruction to match the same string again. For example, the regular expression ( <char>\w)\k<char>, using named groups and backreferencing, searches for adjacent paired characters. When applied to the string I ll have a small coffee, it finds matches in the words I ll, small, and coffee. The metacharacter \w finds any single-word character. The grouping construct ( <char> ) encloses the metacharacter to force the regular expression engine to remember a subexpression match (which, in this case, is any single character) and save it under the name char. The backreference construct \k<char> causes the engine to compare the current character to the previously matched character stored under char. The entire regular expression successfully finds a match wherever a single character is the same as the preceding character. To find repeating whole words, you can modify the grouping subexpression to search for any group of characters preceded by a space instead of simply searching for any single character. You can substitute the subexpression \w+, which matches any group of characters, for the metacharacter \w and use the metacharacter \s to match a space preceding the character group. This yields the regular expression ( <char>\s\w+)\k<char>, which finds any repeating whole words such as the the but also matches other repetitions of the specified string, as in the phrase the theory. This technique never matches the first character of a string, however, because it must be preceeded by white space. To verify that the second match is on a word boundary, add the metacharacter \b after the repeat match. The resulting regular expression, ( <char>\s\w+)\k<char>\b, finds only repeating whole words that are embedded in white space. A backreference refers to the most recent definition of a group (the definition most immediately to the left when matching left to right). Specifically, when a group makes
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