c# print barcode font Match beginning of input Match only numeric digits Match exactly 5 characters Match end of input in .NET

Creating QR in .NET Match beginning of input Match only numeric digits Match exactly 5 characters Match end of input

Match beginning of input Match only numeric digits Match exactly 5 characters Match end of input
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^\d{5}$
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F02NS02
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Figure 2-2 Analysis of a regular expression
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Lesson 1: Validating Input
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Security Alert When validating input, always begin regular expressions with a ^ charac ter and end them with $ . This ensures that input exactly matches the specified regular expression, and does not merely contain matching input.
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Regular expressions can be used to match complex input patterns, too. The following regular expression matches e-mail addresses:
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^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[09]{1,3})(\] )$
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Regular expressions are an extremely efficient way to check user input; however, using regular expressions has a downside:
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Regular expressions are difficult to create unless you are extremely familiar with the format If you have years of Perl programming experience, you won t have any problem using regular expressions in C# code. However, if you have a background in Visual Basic scripting, the cryptic format of regular expressions will seem completely illogical. Creating regular expressions can be confusing, but reading regular expressions definitely is There is a good chance that other programmers will overlook errors in regular expressions when performing a peer code review. The more complex the regular expression, the greater the chance that the structure of the expression contains an error that will be overlooked.
Unfortunately, not all input is as easy to describe as numbers and e-mail addresses. Names and street addresses are particularly difficult to validate because they can con tain a wide variety of characters from international alphabets unfamiliar to you. For example, O Dell, Varkey Chudukatil, Skj naa, Crciun, and McAskill-White are all legit imate last names of real people. Programmatically filtering these examples of valid input from malicious input such as 1 DROP TABLE PRODUCTS -- (a SQL injection attack) is difficult. One common approach is to instruct users to replace characters in their own names. For example, users who normally enter an apostrophe or a hyphen in their names could omit those characters. Users with letters that are not part of the standard Roman alphabet could replace letters with the closest similar Roman character. Although this allows you to more rigorously validate input, it requires users to sacrifice the accurate spelling of their names something many people take very personally. As an alternative, perform as much filtering as possible on the input, and then clean the input of any potentially malicious content. Most input validation should be pessimistic and allow only input that consists entirely of approved characters, but input validation of real names might need to be optimistic and cause an error only when specifically denied characters exist. For example, you could reject a user s name if it contained one
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2
Using Secure Coding Best Practices
of the following characters: !, @, #, $, %, ^, *, (, ), <, >. All these characters are unlikely to appear in a name but likely to be used in an attack. Microsoft Visual Studio .NET provides the following regular expression to match valid names: [a-zA-Z ` \s]{1,40} .
Use Strong Types and Typed Data Comparisons for Non-String Data
User input is generally received in the form of a string. For example, input from an ASP.NET form is received by retrieving the System.Web.UI.WebControls.TextBox.Text string, and input from a Windows Form application is available within the System.Windows.Forms.TextBox.Text string. If the user is not typing text, your application should immediately convert the user input into a stronger type. For example, if you use a TextBox control to allow the user to enter a number from 1 through 100, the following code would convert that input and then validate the range:
C# VB
int myNumber = Int16.Parse(TextBox1.Text); if ( !((myNumber >= 1) && (myNumber <= 100)) ) throw new Exception( Invalid input: Number out of bounds. ); Dim myNumber As Integer = Int16.Parse(TextBox1.Text) If Not ((myNumber >= 1) && (myNumber <= 100)) Then Throw New Exception( Invalid input: Number out of bounds. ) End If
This technique is extremely effective for filtering invalid input, because the .NET Framework throws a format exception when any part of the input string cannot be interpreted as an integer. If the user enters alphabetic characters, a decimal number, or anything other than the characters 0 9 (inclusive), the .NET Framework throws a FormatException with the message Input string was not in a correct format . If the input can be successfully converted but falls outside the inclusive range of 1 100, the exam ple code manually throws an exception. Use similar techniques for validating decimal numbers, currency, dates, and any other input type for which the .NET Framework has a specialized class. The following code verifies that the content of TextBox1 is a valid date, and catches the format exception if the conversion fails:
try { DateTime dt = DateTime.Parse(TextBox1.Text).Date; } catch(FormatException ex) { // Return invalid date message to caller }
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