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copied!<p>It sounds like you want to allow newlines inside the parentheses, but not two or more newlines in a row. A typical regex for that might be </p> <pre><code>'~\(((?:.|\n(?!\r?\n))*)\)~' </code></pre> <p>But that doesn't allow for different kinds of newline (like <code>\r\n</code>, or just <code>\r</code>), nor does it match if there's horizontal whitespace (like spaces or tabs) on the "empty" line. It also doesn't make sure the parentheses are properly balanced; I don't know how important that is to you. Try this regex:</p> <pre><code>'~\(((?:[^\r\n()]|(?:\r\n|[\r\n])(?![ \t]*(?:\r\n|[\r\n]))|(?R))*+)\)~' </code></pre> <p>The first alternative ,<code>[^\r\n()]</code>, matches anything that's not a newline (or part of one) or a parenthesis.</p> <p>If that fails, <code>(?:\r\n|[\r\n])</code> tries to match one of the three kinds of newline, and the negative lookahead <code>(?![ \t]*(?:\r\n|[\r\n]))</code> makes sure the newline isn't followed by another newline, either immediately or with spaces or tabs between them.</p> <p>If the third alternative is reached, the next character should be either an open-paren, in which case <code>(?R)</code> tries to apply the whole regex recursively; or a close-paren, in which case the final <code>\)</code> finishes off the match (or pops up to the next higher level of recursion).</p> <p>Of course, this doesn't account for the possibility of escaped parentheses, Unicode whitespace and line separators, or any number of other refinements, but I'm really just demonstrating how to enforce the <em>no more than one newline</em> rule, and to explain why it's more difficult to do than you might have expected.</p>

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