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    <p>My solution is this:</p> <pre><code>^(([a-zA-Z0-9äöüÄÖÜ]+(-[a-zA-Z0-9äöüÄÖÜ]+)?\s)*([a-zA-Z0-9äöüÄÖÜ]*\.)| (\.[a-zA-Z0-9äöüÄÖÜ]+))|((([a-zA-Z0-9äöüÄÖÜ]*\.)|(\.[a-zA-Z0-9äöüÄÖÜ]+)\s)? ([a-zA-Z0-9äöüÄÖÜ]+(-[a-zA-Z0-9äöüÄÖÜ]+)?\s)*[a-zA-Z0-9äöüÄÖÜ]+ (-[a-zA-Z0-9äöüÄÖÜ]+)?)$ </code></pre> <p><strong>Breaking it down</strong></p> <p>A dot-free word with hyphen matches: Its assumed that a hyphen, if present, is not allowed at the beginning or the end. If this assumption is wrong, it is easy enough to adjust accordingly.</p> <pre><code>[a-zA-Z0-9äöüÄÖÜ]+-[a-zA-Z0-9äöüÄÖÜ]+ </code></pre> <p>A dot-free word without hyphen matches:</p> <pre><code>[a-zA-Z0-9äöüÄÖÜ]+ </code></pre> <p>Thus a dot-free word, with or without a single hyphen (and at most one hypen) matches:</p> <pre><code>[a-zA-Z0-9äöüÄÖÜ]+(-[a-zA-Z0-9äöüÄÖÜ]+)? </code></pre> <p>We can make use of a general design pattern (no pun intended), to get exactly one X with any number of Y:</p> <pre><code>(Y*X)|(XY+) </code></pre> <p>So applying this rule, a dotted word with exactly one dot matches:</p> <pre><code>([a-zA-Z0-9äöüÄÖÜ]*\.)|(\.[a-zA-Z0-9äöüÄÖÜ]+) </code></pre> <p>Similarly get a stream of words with exactly one dotted, we slightly modify the general rule for a space separator. So a stream of words with exactly one dotted matches:</p> <pre><code> ((Y\s)*X)|(X\s(Y\s)*Y) </code></pre> <p>where: 1. Y = the regex for a dot-free word 2. X = the regex for a dotted word </p> <p>Similarly a stream of only dot-free words would match:</p> <pre><code>(Y\s)*Y </code></pre> <p>where Y is as before.</p> <p>Combining the two meta-regexes, a stream of words with at most one dotted word matches:</p> <pre><code>((Y\s)*X)|((X\s)?(Y\s)*Y) </code></pre> <p>where X and Y are as before.</p> <p>The final step is to substitute X and Y back in the preceding meta-regex to yield my propoosed solution. The really nice thing about my solution is that it does not use look-aheads, so resolution is faster and it works on all flavours of regex, including even the very primitive flavours from XML Schema, XPATH and XSLT.</p> <p>Add ^ and $ at the start and end if needed.</p>
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