Equal Sign in Proxypass Regular Expression Replacement String

Recognizing that I may be misusing the proxypass irule, assuming I want to replace a string matched by a regular expression with another string that contains an equal sign, how would I protect the equal sign from being the terminator for the replacement string? I've tried other delimiters, TCL backslash and bracket protection and HTML %3D, to no avail. E.g.,

/ := =^(.*?)/$=${1}/foo.jsp?var1{=}value1&var2{=}value2&=

Where "{=}" should be replaced with a literal equal sign.

Thanks.