equivalent for backreference \n

Hello,

obviously the regular expressions in procmail are similar as those
of egrep. So it seems there is no backreference \n.

I want to use something like [0-9]\1\1 to match a sequence of three
equal digits. This should match 111 and 222 but not 122.

Is there something equivalent in procmail/egrep?

-Hanspeter
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