LUCENE-4078: PatternReplaceCharFilter assertion error caused by malformed utf-16. This is most likely a bug in the JDK because a Pattern("").replaceAll("x") is inserted in between surrogate pair characters and corrupts strings. A temporary fix is to check for this at random pattern generator and pick again if detected.
lucene/dev/trunk/lucene/analysis/common/src/test/org/apache/lucene/analysis/pattern/TestPatternReplaceCharFilter.java