25.2.3. Quoting special characters

As of Apache 1.3.20, special characters in TestString and Substitution strings can be escaped (that is, treated as normal characters without their usual special meaning) by prefixing them with a slosh ('') character. In other words, you can include an actual dollar-sign character in a Substitution string by using '$'; this keeps mod_rewrite from trying to treat it as a backreference.