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