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copied!<p>Your non-trivial regex: <code>^[a-zA-Z]([-\s\.a-zA-Z]*('(?!'))?[-\s\.a-zA-Z]*)*$</code>, is better written with comments in free-spacing mode like so:</p> <pre class="lang-cs prettyprint-override"><code>Regex re_orig = new Regex(@" ^ # Anchor to start of string. [a-zA-Z] # First char must be letter. ( # $1: Zero or more additional parts. [-\s\.a-zA-Z]* # Zero or more valid name chars. ( # $2: optional quote. ' # Allow quote but only (?!') # if not followed by quote. )? # End $2: optional quote. [-\s\.a-zA-Z]* # Zero or more valid name chars. )* # End $1: Zero or more additional parts. $ # Anchor to end of string. ",RegexOptions.IgnorePatternWhitespace); </code></pre> <p>In English, this regex essentially says: <em>"Match a string that begins with an alpha letter <code>[a-zA-Z]</code> followed by zero or more alpha letters, whitespaces, periods, hyphens or single quotes, but each single quote may not be immediately followed by another single quote."</em></p> <p>Note that your above regex allows oddball names such as: <code>"ABC---...'... -.-.XYZ "</code> which may or may not be what you need. It also allows multi-line input and strings that end with whitespace.</p> <p>The "infinite loop" problem with the above regex is that <a href="http://www.regular-expressions.info/catastrophic.html" rel="nofollow">catastrophic backtracking</a> occurs when this regex is applied to a long invalid input which contains two single quotes in a row. Here is an equivalent pattern which matches (and fails to match) the exact same strings, but does not experience catastrophic backtracking:</p> <pre class="lang-cs prettyprint-override"><code>Regex re_fixed = new Regex(@" ^ # Anchor to start of string. [a-zA-Z] # First char must be letter. [-\s.a-zA-Z]* # Zero or more valid name chars. (?: # Zero or more isolated single quotes. ' # Allow single quote but only (?!') # if not followed by single quote. [-\s.a-zA-Z]* # Zero or more valid name chars. )* # Zero or more isolated single quotes. $ # Anchor to end of string. ",RegexOptions.IgnorePatternWhitespace); </code></pre> <p>And here it is in short form in your code context:</p> <pre class="lang-cs prettyprint-override"><code>const string PATTERN = @"^[a-zA-Z][-\s.a-zA-Z]*(?:'(?!')[-\s.a-zA-Z]*)*$"; </code></pre>

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