Regular Expressions in .NET Regular Expressions in .NET By: Nasser - PowerPoint PPT Presentation

Regular Expressions in .NET Regular Expressions in .NET By: Nasser Alshammari College of Science, Department of Computer Science Old Dominion University

Outline Outline  Regular Expressions?  Why do we need them?  RE language  RE in .NET  Conclusion

Regular Expressions? Regular Expressions?  Regular Expressions (regex) are a special string that describes a search pattern.  Provide flexible and easy string matching.  Syntax:  POSIX BRE  POSIX ERE  POSIX character classes  Example:  \b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b

Why do we need them? Why do we need them?  Security  Buffer overflow attack.  SQL injection attack.  Cross-site scripting attack.  Easy and powerful  Flexibility

RE language RE language  Metacharacters: a character that has a special meaning. . Any single character. a.c matches abc [ ] A single character contained within the [abc] matches ”a”, ”b”, or ”c” brackets. [a-z] matches any characater in this range [^ ] A single character that is not contained [^abc] matches any character other than ”a”, within the brackets. ”b”, or ”c”. ^ Starting position of the string $ Ending position of the string BRE \( \) Defines a group or subexpression ERE ( ) * Matches the preceding element ab*c matches ”ac”, ”abc”, ”abbc”, etc zero or more times BRE \{m,n\} Matches the preceding element at a{3,5} matches only ”aaa”, ”aaaa”, and ERE {m,n} least m and not more than n times ”aaaaa”

Continue. Continue.  More Examples .at matches any three-character string ending with "at", including "hat", "cat", and "bat". [hc]at matches "hat" and "cat". [^b]at matches all strings matched by .at except "bat". ^[hc]at matches "hat" and "cat", but only at the beginning of the string or line. [hc]at$ matches "hat" and "cat", but only at the end of the string or line. \[.\] matches any single character surrounded by "[" and "]" since the brackets are escaped, for example: "[a]" and "[b]".

Continue. Continue.  POSIX ERE Metacharacters ? Matches the preceding element zero or one time. For example, ba? matches "b" or "ba". + Matches the preceding element one or more times. For example, ba+ matches "ba", "baa", "baaa", and so on. | The choice (aka alternation or set union) operator matches either the expression before or the expression after the operator.  Examples [hc]+at matches "hat", "cat", "hhat", "chat", "hcat", "ccchat", and so on, but not "at". [hc]?at matches "hat", "cat", and "at". cat | dog matches "cat" or "dog".

Continue. Continue.  POSIX character classes Many ranges of characters depend on the locale settings (i.e., in some  settings letters are organized as abc...zABC...Z, while in some others as aAbBcC...zZ) POSIX .NET ASCII Description [:alnum:] [A-Za-z0-9] Alphanumeric characters [:word:] \w [A-Za-z0-9_] Alphanumeric characters plus "_" [:alpha:] [A-Za-z] Alphabetic characters [:blank:] [ \t] Space and tab [:digit:] \d [0-9] Digits [:space:] \s [ \t\r\n\v\f] Whitespace characters

RE in .NET RE in .NET  .NET's regex flavor is feature-rich.  RegularExpressionValidator Server Control.  System.Text.RegularExpressions Namespace.  Examples: ^[a-zA-Z''-'\s]{1,40}$ Ravi Mukkamala Allows uppercase or lowercase letters O'Dell or whitespaces up to 40 characters. ^\d{3}-\d{2}-\d{4}$ 000-11-2222 Allows only digits separated by hyphens. ^\d+$ 0 Positive integer greater than zero. 123 ^\d+(\.\d\d)?$ 5.00 Positive currency amount. If there is a decimal point, it requires 2 numeric characters after the decimal point. For example, 3.00 is valid but 3.1 is not. ^(-)?\d+(\.\d\d)?$ -2.44 Positive or negative currency.

Continue. Continue.  Using the RegularExpressionValidator Server Control <%@ language="C#" %> <form ID="form1" runat="server"> <asp:TextBox ID="txtName" runat="server"/> <asp:Button ID="btnSubmit" runat="server" Text="Submit" /> <asp:RegularExpressionValidator ID="regexpName" runat="server" ErrorMessage="This expression does not validate." ControlToValidate="txtName" ValidationExpression="^[a-zA-Z'.\s]{1,40}$" /> </form>

Continue. Continue.  Using System.Text.RegularExpressions  The Regex class  IsMatch : returns true if a match is found.  Match : returns a Match object if a match is found.  Matches : returns a MatchCollection object.  Replace : replaces a matched string with another.  Split : splits a string according to regex and retuens String[]. // Instance method: Regex reg = new Regex(@"^[a-zA-Z'.]{1,40}$"); Response.Write(reg.IsMatch(txtName.Text)); // Static method: if (!Regex.IsMatch(txtName.Text, @"^[a-zA-Z'.]{1,40}$")) { // Name does not match }

Continue. Continue.  Regex.Replace public class Example { public static void Main() { string input = "This is text with far too much " + "whitespace."; string pattern = "\s+"; string replacement = " "; Regex rgx = new Regex(pattern); string result = rgx.Replace(input, replacement); Console.WriteLine("Original String: {0}", input); Console.WriteLine("Replacement String: {0}", result); } // The example displays the following output: // Original String: This is text with far too much whitespace. // Replacement String: This is text with far too much whitespace.

Continue. Continue.  Regex.Split string input = @"07/14/2007"; string pattern = @"(-)|(/)"; Regex regex = new Regex(pattern); foreach (string result in regex.Split(input)) { Console.WriteLine("'{0}'", result); } // Under .NET 1.0 and 1.1, the method returns an array of // 3 elements, as follows: // '07' // '14' // '2007' // Under .NET 2.0, the method returns an array of // 5 elements, as follows: // '07' // '/' // '14' // '/' // '2007'

Continue. Continue.  Comments Regex regex = new Regex(@" ^ # anchor at the start (?=.*\d) # must contain at least one numeric character (?=.*[a-z]) # must contain one lowercase character (?=.*[A-Z]) # must contain one uppercase character .{8,10} # From 8 to 10 characters in length \s # allows a space $ # anchor at the end", RegexOptions.IgnorePatternWhitespace);

Continue. Continue.  Match class: immutable and has no public constructor. // Search for a pattern that is not found in the input string. string pattern = "dog"; string input = "The cat saw the other cats playing in the back yard."; Match match = Regex.Match(input, pattern); if (match.Success ) // Report position as a one-based integer. Console.WriteLine("'{0}' was found at position {1} in '{2}'.", match.Value, match.Index + 1, input); else Console.WriteLine("The pattern '{0}' was not found in '{1}'.",pattern, input);

Continue. Continue.  MatchCollection class: immutable and has no public constructor. static void Main(string[] args) { string input = "The quick brown dog jumps over the lazy dog"; string pattern = "dog"; MatchCollection matches = Regex.Matches(input, pattern); foreach (Match match in matches) Console.WriteLine("'{0}' found at position '{1}'", match.Value, match.Index); } Output: 'dog' found at position '16' 'dog' found at position '40'

Continue. Continue.  More Examples // Create a Regex that accepts all URLs containing the host fragment www.odu.edu. Regex myRegex = new Regex(@"http://www\.odu\.edu/.*"); //a WebPermission that gives permissions to all the hosts containing the same host fragment. WebPermission myWebPermission = new WebPermission(NetworkAccess.Connect, myRegex); // Check whether all callers higher in the call stack have been granted the permission. myWebPermission.Demand();

Conclusion Conclusion  Regex is your friend, use it.  Regex offers additional security measures.  .NET supports many of regex features. Thank You

References References  http://www.regular-expressions.info  http://msdn.microsoft.com