You have either constructed a regular expression dynamically from your code or based on user input. You need to test the validity of this regular expression’s syntax before you actually use it.
Use the following method to test the validity of a regular expression’s syntax:
using System; using System.Text.RegularExpressions; public static bool VerifyRegEx(string testPattern) { bool isValid = true; if ((testPattern != null) && (testPattern.Trim( ).Length > 0)) { try { Regex.Match("", testPattern); } catch (ArgumentException) { // BAD PATTERN: Syntax error isValid = false; } } else { //BAD PATTERN: Pattern is null or blank isValid = false; } return (isValid); }
To use this method, pass it the regular expression that you wish to verify:
public static void TestUserInputRegEx(string regEx) { if (VerifyRegEx(regEx)) Console.WriteLine("This is a valid regular expression."); else Console.WriteLine("This is not a valid regular expression."); }
The VerifyRegEx
method calls the static
Regex.Match
method,
which is useful for running quick regular expressions against a
string. The static Regex.Match
method returns a
single Match
object. By using this static method
to run a regular expression against a string (in this case a blank
string), we can determine whether the regular expression is invalid
by watching for a thrown exception. The
Regex.Match
method will throw an
ArgumentException
if the regular expression is not
syntactically correct. The Message
property of
this exception contains the reason the regular expression failed to
run, and the ParamName
property contains the
regular expression passed to the Match
method.
Both of these properties are read-only.
Before testing the regular expression with the static
Match
method, the regular expression is tested to
see if it is null
or blank. A
null
regular expression string returns an
ArgumentNullException
when passed in to the
Match
method. On the other hand, if a blank
regular expression is passed in to the Match
method, no exception is thrown (as long as a valid string is also
passed to the first parameter of the Match
method).