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    <h3>Assumption</h3> <p>A match is defined as a sequence:</p> <ul> <li>Starting from the first <code>"</code> in the input string. Let us call this the 1st <code>"</code> or the opening <code>"</code>.</li> <li>There is no <code>;</code> before the opening <code>"</code></li> <li><code>;</code> is allowed in the sequence, if the last <code>"</code> in the sequence is odd, counting from the 1st <code>"</code>.</li> <li><code>;</code> is not allowed in the sequence, if the last <code>"</code> in the sequence is even, counting from the 1st <code>"</code>.</li> <li>Ends with the furthest <code>"</code> that satisfies the conditions above.</li> </ul> <h3>Solution</h3> <p>Find the <em>first</em> match with this regex (raw form):</p> <pre> ^(?>[^;"]*)"((?>(?>"[^";]*(?="[^"]*$)|"[^";]*"|[^"]*)+))" </pre> <p>In C# string literal:</p> <pre><code>"^(?&gt;[^;\"]*)\"((?&gt;(?&gt;\"[^\";]*(?=\"[^\"]*$)|\"[^\";]*\"|[^\"]*)+))\"" </code></pre> <p>The result will be in the first capturing group.</p> <h3>Explanation</h3> <p>Syntax explanation:</p> <ul> <li><code>(?&gt;pattern)</code> is non-backtracking/possessive sub-expression. It prevents the engine from backtracking. This is a form of optimization.</li> <li><code>(?=pattern)</code> is zero-width positive look-ahead. It checks that the string ahead conforms to the <code>pattern</code> without consuming the text.</li> <li><code>|</code> is alternation. The thing I want to note here is that regex-directed engine will check each rule from left to right, and will not consider other rules if a match is found. That means the order matters in determining the matches.</li> <li>I assume you are familiar with the rest, since they are quite basic.</li> </ul> <p>For the sake of explanation, I will use the raw regex, with the non-backtracking optimization removed:</p> <pre> ^[^;"]*"((?:"[^";]*(?="[^"]*$)|"[^";]*"|[^"]*)+)" </pre> <p>Due to the requirement "first and last quote in a line", there is at most 1 match per line.</p> <p>By doing some analysis on the requirement, we know that the text <em>before</em> the interested portion should not contain <code>;</code> (part of the requirement) or <code>"</code> (otherwise, the quote will not be the first). Therefore, we can write <code>^[^;"]*</code> to anchor the match from the start of the string and match everything up until the first quote <code>"</code>.</p> <p>This is the quoted string part, broken up for ease of explanation:</p> <pre> " ( (?: "[^";]*(?="[^"]*$) | "[^";]*" | [^"]* )+ ) " </pre> <p>Let us focus on these 3 fragments, and I will start explaining from bottom-up:</p> <pre> "[^";]*(?="[^"]*$) "[^";]*" [^"]* </pre> <p>For all cases here, the last quote we encountered is always an odd quote.</p> <ul> <li><code>[^"]*</code>: The last quote is ensured to be odd quote, so we can have anything including <code>;</code>, but except for <code>"</code>.</li> <li><code>"[^";]*"</code>: The last quote is odd quote, and after this is also odd quote. This part deals with the portion after an even quote, where <code>;</code> is disallowed.</li> <li><code>"[^";]*(?="[^"]*$)</code>: This is the tricky part, which deals with the case where the string has an odd number of quotes (>= 3). I made sure that after the even quote, there is no <code>;</code> and followed by the last quote <code>"</code> in the string. <em>"Followed by the last quote <code>"</code> in the string"</em> is achieved with the look-ahead <code>(?="[^"]*$)</code>.</li> </ul> <p>The fragment <code>"[^";]*(?="[^"]*$)</code> must be placed in front of <code>"[^";]*"</code> to avoid backtracking, so that we can apply the non-backtracking optimization.</p>
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    1. COYour solution works fine, but on multiline it matchs all line until next semicolon. In my case the mach must end with the last quotes of the line!
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    2. CO@San: Just read the file line by line, or add `\n` into all the character classes.
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    3. CO^[^;"\n]*"((?:"[^";\n]*(?="[^"\n]*$)|"[^";\n]*"|[^"\n]*)+)" works perfect, thanks
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