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    <p>In order to do such a thing, you could use negative look ahead before matching a new group.</p> <p>I will use a more general PCRE notation:</p> <pre><code>(.)((?!\1).)((?!\1|\2).)\3\2((?!\1\2\3).)\2\1 </code></pre> <p>The regex above will match the string <code>12332421</code> but will not match <code>12112421</code> or <code>11111111</code>.</p> <p>A short explanation:</p> <pre><code>(.) // match any character (except line breaks) and store it in group 1 ( // open group 2 (?!\1) // if looking ahead group 1 cannot be seen, . // match any character (except line breaks) ) // close group 2 ( // open group 3 (?!\1|\2) // if looking ahead group 1 or 2 cannot be seen, . // match any character (except line breaks) ) // close group 3 \3 // back referencing group 3 \2 // back referencing group 2 ( // open group 4 (?!\1\2\3) // if looking ahead group 1, 2 or 3 cannot be seen, . // match any character (except line breaks) ) // close group 4 \2 // back referencing group 2 \1 // back referencing group 1 </code></pre> <p>Of course, you don't really need to group #4 since you're not back referencing it.</p> <p>You might agree with me that regex is not be the best tool to do this kind of matching...</p> <p><strong>Edit</strong>:</p> <p>Well, I don't see how you're going to build these regexes, but can't imagine it being easier than this little method which simply accepts a pattern and a target string and tests if they match:</p> <pre><code>public class Test { public static boolean matchesPattern(String text, String pattern) { if(text.length() != pattern.length()) return false; Map&lt;Character, Character&gt; mappings = new HashMap&lt;Character, Character&gt;(); for(int index = 0; index &lt; pattern.length(); index++) { Character patternChar = pattern.charAt(index); Character textChar = text.charAt(index); if(mappings.containsKey(patternChar)) { if(mappings.get(patternChar) != textChar) return false; } else { if(mappings.values().contains(textChar)) return false; mappings.put(patternChar, textChar); } } return true; } public static void main(String[] args) { String pattern = "abccbdba"; String[] tests = {"12332421", "12112421", "11111111"}; for(String t : tests) { System.out.println(t+" -&gt; "+matchesPattern(t, pattern)); } } } </code></pre> <p>which produces the following output:</p> <pre><code>12332421 -&gt; true 12112421 -&gt; false 11111111 -&gt; false </code></pre>
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    1. POCan one use .NET RegEx backreference in a character class to exclude previously matched characters?
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    1. USBart Kiers
    1. USBart Kiers
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    1. COYou mean unrolling? Which is the same problem as with matching parentheses? .NET supports this explicitly with balancing groups. But also consider the look forward / look backward solution I provided, it may work if I got the problem domain correct.
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    2. COI figured regex would be best because in addition to matching a whole word, it could find the pattern as a substring which would require a very convoluted amount of logic to do by matching charAt idx 0 with charAt idx 3, but not equal to charAt idx 1 and 2, etc. Nice solution, btw.
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    3. COThanks Roly. Also see my edit.
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