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copied!<p>A Regular Expression (Regex), as you will find out, is a tool used in matching text against a pattern and even extracting or replacing matches in the source text.</p> <p>To do this the Regex engine (which exists in the .Net framework inside the namespace System.Text.RegularExpressions) uses some established patterns that represent certain kinds of chars.</p> <p>To use a Regex, you pass it the pattern agains which a text will be tested and the text itself.</p> <p>For instance, the following snippet tests if there are lowercase letters in the text:</p> <pre><code>using System.Text.RegularExpressions; ... var Pattern = "[a-z]"; if (Regex.IsMatch(SomeText, Pattern)) { //... there is at least one lower case char in the text } </code></pre> <p>The pattern used above estates that we are interested in a range of chars from lowercase "a" to lowercase "z".</p> <p>A pattern to require at least one lowercase letter, one digit and one uppercase letter could probably be something like</p> <pre><code>@"[\d]+[a-z]+[A-Z]|[\d]+[A-Z]+[a-z]|[a-z]+[\d]+[A-Z]|[a-z]+[A-Z]+[\d]|[A-Z]+[\d]+[a-z]|[A-Z]+[a-z]+[\d]" </code></pre> <p>As you can see, a Regex pattern can be as simple as in the first example, but may escalate fast in complexity depending on the kind of information you want to check or extract.</p> <p>In the Regex above, the items between square brackets are called character classes. The plus sign after an element is a quantifier, it indicates that the item may appear one or more times. Items separated by a vertical bar indicate alternatives. The "\d" pattern represents any digit from "0" to "9".</p> <p>You don't really need a Regex to check the password strength in your application, but using then instead of the explicit tests you are currently making would (in my opinion), greatly improve your program's readability:</p> <pre><code>using System.Text.RegularExpressions; ... if (!Regex.IsMatch(password, "[A-Z]")) { errorMessage = "Your password must contain at least one uppercase character."; } else if (!Regex.IsMatch(password, "[a-z]") { errorMessage = "Your password must contain at least one lowercase character."; } else if (! Regex.IsMatch(password, @"[\d]")){ errorMessage = "Your password must contain at least one numerical character."; } ... </code></pre>

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