code: public StrSubstitutor setVariablePrefix(final String prefix) {
         if (prefix == null) {
              throw new IllegalArgumentException("Variable prefix must not be null!");
          }
          return setVariablePrefixMatcher(StrMatcher.stringMatcher(prefix));
      }
docs: Sets the variable prefix to use.
  <p>
  The variable prefix is the character or characters that identify the
  start of a variable. This method allows a string prefix to be easily set.
  
  @param prefix  the prefix for variables, not null
  @return this, to enable chaining
  @throws IllegalArgumentException if the prefix is null