How to replace value between xml tags using regex in Ant?

I'm trying to use Ant for replacing value in xml.

My xml file:

<session-config>
    <session-timeout>60</session-timeout>
</session-config>

I want to replace 60 to 20

And use for this following regexp in ant task replaceregexp:

(?<=session-timeout>)[\S\s]*?(?=</session-timeout)


<target name="step1">
        <replaceregexp file="WEB-INF/web.xml"
                       byline="true"
                       match="((?<=session-timeout\>)[\S\s]*?(?=\<\/session-timeout))"
                       replace='20'/>
</target>

But got fatal error from ant after execution:

[Fatal Error] The value of attribute "match" associated with an element type "replaceregexp"
 must not contain the '<' character.

Please advise, how to change my regexp, or maybe there is an another solution of this problem? Thanks.