@Override
        public int hashCode() {
            int hash = 3;
            hash = 41 * hash + Objects.hashCode(expression);
            hash = 41 * hash + Objects.hashCode(replacement);
            return hash;
        }
Apply the rule against the input string, returning the modified
string or null if the rule didn't apply (and no modifications were
made)

@param input the input string
@return the modified string if this rule applied, or null if the
input was not modified by this rule
generator-parent/generator-translator/src/main/java/com/speedment/generator/translator/util/Pluralis.java