The following simple expression with replaceAll and a regex (to remove tags like <mytag>):

.setProperty("testString", new SimpleExpression("${exchangeProperty.testString.replaceAll('<[^>]+>','')}"))

Causes an IllegalArgumentException. It appears that the presence of the '<' character at position 0 causes the DefaultClassResolver to throw the IllegalArgumentException when testing the parameter type. This is the method that does the loadClass:

https://github.com/apache/camel/blob/4c7ce615ab464056d1d8cc34503040542b91e896/core/camel-base-engine/src/main/java/org/apache/camel/impl/engine/DefaultClassResolver.java#L65

called from:

https://github.com/apache/camel/blob/4c7ce615ab464056d1d8cc34503040542b91e896/components/camel-bean/src/main/java/org/apache/camel/component/bean/MethodInfo.java#L646

This same expression worked in Camel 2.x, but fails in Camel 3.14.x. Workarounds are available (do the regex/replace in a processor, for example).