Sed search and replace on variable string -

i'm trying search , replace string in file on mac terminal using sed. i'm able search , replace simple string:

sed -i.bak 's/hosts/boasts/g' file.txt 

but i'm trying on little more complicated, string want replace looks 'hosts:"123.123.123.123, 12345"' - 123.123.123.123 being variable ip can't search that, i'm trying use regular expressions, "." indicate don't know ip address be.

i've tried following no luck:

sed -i.bak 's/hosts:"., 00000"/hosts:"999.999.999.999, 00000"/g' file.txt 

you try following:

echo "hosts:\"123.123.123.123, 12345\"" | sed -e 's/[0-9][0-9][0-9]\.[0-9][0-9][0-9]\.[0-9][0-9][0-9]\.[0-9][0-9][0-9]/999.999.999.999/g'. 

each [0-9] digit, , each \. actual symbol, not "match character" symbol on sed. assumes ips have structure. if you're dealing xxx.xxx.xxx.xxx:xxxx you'll have edit accordingly.