If I use trigger "

{hostname:web.get.page[site].str(substr)}

" then substring found only if it occurs in the first 256 (not sure) bytes of value including http header.
I did not found this limitation in documentation, and it works OK in zabbix 1.8.8.
Item "hostname:web.get.regex[site,,,substr]" works good on both versions, but sometimes it's useful to get full page content in alert (as value).
As workaround I use trigger like this:

{hostname:web.page.get[site].str("substr")}

<1&

{hostname:web.page.regexp[site,,,substr].count(#2,"substr","eq")}

<1
first part five value for alerter, second part set real condition.