echo ${var:position1:lenght}

s='myspecificBhost.fqdn.com myspecificaBhost.fqdn.com myspecificzBhost.fqdn.com'
[[ "$s" =~ (B[a-z][a-z][a-z][a-z]) ]] && echo "${BASH_REMATCH[1]}"
Bhost

sed -nr '/.*c(.{4,6}).*/s//1/p' input.txt | cut -c2-6
RESULT:
Bhost

cat input.txt | grep -o B.... | head -1
RESULT:
Bhost

cat file | grep -o B....

text='myspecificBhost.fqdn.com myspecificaBhost.fqdn.com myspecificzBhost.fqdn.com'

text=${text#*B}
text=${text:0:4}
echo "${text}"

host

echo "B${text}"