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Regular Expressions in Visual Basic, C++ and C# under .NET

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Regular Expressions in .NET

Visual Basic, C++ and C# amongst other languages can make use of the shared regular expression library provided under Microsoft's .NET framework. The library is uses Perl-flavoured regular expressions, but with some subtle variations.

Operator Type Examples Description Literal Characters
Match a character exactly a A y 6 % @ Letters, digits and many special
characters match exactly \$ \^ \+ \\ \? Precede other special characters
with a \ to cancel their regex special meaning \n \t \r Literal new line, tab, return \cJ \cG Control codes \xa3 Hex codes for any character \u00a3 Unicode for any character Anchors and assertions ^ Starts with $ Ends with \b \B on a word boundary,
NOT on a word boundary Character groups
any 1 character from the group [aAeEiou] any character listed from [ to ] [^aAeEiou] any character except aAeEio or u [a-fA-F0-9] any hex character (0 to 9 or a to f) . any character at all
(not new line in some circumstances) \s any space character (space \n \r or \t) \w any word character (letter digit or _) \d any digit (0 through 9) \S \W \D any character that is NOT a space
word character or digit Counts
apply to previous element + 1 or more ("some") * 0 or more ("perhaps some") ? 0 or 1 ("perhaps a") {4} exactly 4 {4,} 4 or more {4,8} between 4 and 8 Add a ? after any count to turn it sparse (match as few as possible) rather than have it default to greedy Alternation | either, or Grouping ( ) group for count and save to variable (?: ) group for count but do not save

Option modes can be specified as using a (?x ) if you wish them to cover part of the regular expression, or as per the following table if you want them to apply to the whole of a regular expression:

Option Description .IgnoreCase Ignore case in matching .IgnorePatternWhiteSpace White space is to be treated as a comment (otherwise it matches exactly) .Singleline . to match everything including new line (otherwise it matches everything except new line) .Multiline ^ and $ to match embedded new lines

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Comment: "Mostly excellent; however, I see these potential problems ..."
Visitor Ranking 4.0 (5=excellent, 1=poor)
Comment by Yahya Abdal-Aziz

(published 2010-05-18)

Mostly excellent; however, I see these potential problems for new regex users:

1. Is space a literal character i.e. matching "[ ]" or some special character e.g. "\x" ?

2. What do you mean by "a word boundary"?

3. If counts "apply to previous element", what defines, bounds or marks an element?

4. You write: "Add a ? after any count to turn it sparse". Would that be, e.g., "{4,8?}" or "{4,8}?" ?

[#3563]

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