Regular expression:
^\s?Line\s{1}\d{1,}\s+[\w_\d]+\s+.*

^\s? ==> Starting with Zero or only one space
\s{1} ==> Single space after "Line"
\d{1,}==> Follwed by minimum one digit(maxm any no of digits)
\s+ ==> Follwed by one or more number of spaces
[\w_\d]+ ==> Follwed by any combination words,underscores and digits
\s+ ==> Follwed by 1 or more number of spaces
.* ==> Follwed by rest of the string

Regards,
Gytaso