[Exim] Extra \\ in filter help

OK - I'm getting close here. The following expression works perfectly when
tested with exim -be

${sg\
    {${sg\
       {${readfile{/etc/exim/blockfrom}{|}}}\
       {\\|*#end.*|#.*?\\|}\
       {}\
       }}\
    {(\\|)\\|}\
    {$1}\
}

but - when I try this: in a filter:

if "$h_from:" mateches "${sg\
    {${sg\
       {${readfile{/etc/exim/blockfrom}{|}}}\
       {\\|*#end.*|#.*?\\|}\
       {}\
       }}\
    {(\\|)\\|}\
    {$1}\
}"

It complains about regular expression errors. I did read that extra \\ are
neaded, but I don't know how many and where.

What characters in the above expression need extra \ and how many?