[cisco-voip] Voice translation rule help

[cisco.voip]

I am having a problem understand one piece of the voice trans rules -
the "\" = escape the special meaning of the next character - what does this
mean?

For example - I do not understand the use of the \2 in the following
example:

rule 1 /^\(555\)+\(.*\)/ /444\2/

rt1(cfg-translation-rule)#do test voice trans 1 5551212
Matched with rule 1
Original number: 5551212        Translated number: 4441212

Thanks