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    <p>It looks like you can use <code>MAN(?!.*PN)</code>. This matches <code>MAN</code> and uses negative lookahead to make sure that it's not followed by <code>PN</code> (<a href="http://www.rubular.com/r/BP0T80KVsR" rel="nofollow noreferrer">as seen on rubular.com</a>).</p> <p>Given <code>MAN_PN_MAN_BLEH</code>, the above pattern will find the second <code>MAN</code>, since it's not followed by <code>PN</code>. If you want to validate the entire string and make sure that there's no <code>MAN.*PN</code>, then you can use something like <code>^(?!.*MAN.*PN).*MAN.*$</code> (<a href="http://www.rubular.com/r/QtlvGlfisu" rel="nofollow noreferrer">as seen on rubular.com</a>).</p> <h3>References</h3> <ul> <li><a href="http://www.regular-expressions.info/lookaround.html" rel="nofollow noreferrer">regular-expressios.info/Lookarounds</a></li> </ul> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/3077566/how-can-i-check-if-every-substring-of-four-zeros-is-followed-by-at-least-four-one/3080023#3080023">How can I check if every substring of four zeros is followed by at least four ones using regular expressions?</a></li> </ul> <hr> <h3>Non-regex option</h3> <p>If the strings are to be matched literally, then you can also check for indices of substring occurrences.</p> <p>In Python, <code>find</code> and <code>rfind</code> return lowest and highest index of substring occurrences respectively.</p> <p>So to make sure that <code>string1</code> occurs but never followed by <code>string2</code>, and both returns <code>-1</code> if the string is not found, so it looks like you can just test for this condition:</p> <pre><code>string.rfind(s, string2) &lt; string.find(s, string1) </code></pre> <p>This compares the leftmost occurrence of <code>string1</code> and the rightmost occurrence of <code>string2</code>.</p> <ul> <li>If neither occurs, both are <code>-1</code>, and result is <code>false</code></li> <li>If <code>string1</code> occurs, but <code>string2</code> doesn't, then result is <code>true</code> as expected</li> <li>If both occurs, then the rightmost <code>string2</code> must be to the left of the leftmost <code>string1</code> <ul> <li>That is, no <code>string1</code> is ever followed by <code>string2</code></li> </ul></li> </ul> <h3>API links</h3> <ul> <li><a href="http://docs.python.org/library/string.html#string.find" rel="nofollow noreferrer"><code>find</code></a></li> <li><a href="http://docs.python.org/library/string.html#string.rfind" rel="nofollow noreferrer"><code>rfind</code></a></li> </ul>
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    1. COIn the intended example, it would not be a match. So, if string1 occurs, but is ever followed by string2, it would negate the find.
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    2. CO@apalopohapa: I'm still not sure if I get this 100% correct, so feel free to edit the question later with more examples and/or unaccept my answer if it fails you on some input. I will come back to this several hours from now and see if there are any unresolved issues.
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    3. CO@apalopohapa: Also, I think `MAN(?!.*P(?=.*N?))` is just `MAN(?!.*P)`, which then opens up possibilities of `MAN[^P]*` etc. The more information we have, the better answers we can give.
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