In ASP.NET Web Forms, which validator control checks whether the entered data matches a specific pattern or format defined by a regular expression, such as an email address format?

Introduction / Context:
Input validation often involves checking not just that a value exists, but that it follows a specific pattern, such as email, postal code, or phone number formats. ASP.NET Web Forms includes a validator control that uses regular expressions to enforce such patterns. Interviewers commonly ask which validator handles pattern based validation because it demonstrates familiarity with built in validation tools.

Given Data / Assumptions:

• We are validating data in a Web Forms input control.
• The data must match a particular format, such as a regular expression for an email address.
• The question asks which validator is designed for pattern matching.

Concept / Approach:
The RegularExpressionValidator control checks whether the value of the associated control matches a regular expression defined in its ValidationExpression property. If the value matches, the validator passes; if it does not, validation fails and an error message is displayed. This allows developers to encode complex validation rules in a concise, reusable pattern format.

Step-by-Step Solution:
Step 1: Add an input control such as a TextBox to the page for user input, for example TextBoxEmail. Step 2: Add a RegularExpressionValidator to the page and set its ControlToValidate property to TextBoxEmail. Step 3: Set the ValidationExpression property to a regular expression that represents the desired format, such as a pattern for email addresses. Step 4: Configure the ErrorMessage property with a clear message, for example "Please enter a valid email address." Step 5: When the user submits the form, ASP.NET evaluates the regular expression against the input and flags the control as invalid if the pattern does not match.

Verification / Alternative check:
You can verify the behaviour by entering valid and invalid values that match or violate the regular expression. For example, a proper email format like user@example.com should pass, whereas plain text without an at symbol should fail. RegularExpressionValidator is documented as the control that uses regular expressions for validation, confirming that it is the correct answer for pattern based checks.

Why Other Options Are Wrong:
Option A is wrong because CompareValidator compares values between controls or against constants; it does not evaluate regular expressions. Option C is incorrect because CustomValidator allows you to write any custom logic, but it does not provide built in regular expression handling. Option D is wrong because RangeValidator enforces minimum and maximum bounds, not pattern based formats.

Common Pitfalls:
Common pitfalls include using overly strict or overly loose regular expressions that either reject valid data or accept invalid inputs. Another issue is forgetting to escape special characters in patterns or not testing against a diverse set of sample values. Developers should also remember that client side validation can be bypassed, so server side checks remain essential. Using RegularExpressionValidator correctly helps enforce consistent data formats with minimal code.

Final Answer:
The validator that checks whether entered data matches a specific pattern or format is the RegularExpressionValidator control in ASP.NET Web Forms.