var sql = "Select * from table b left join Lkp_table2 a on a.col=b.col";

// Regex to find first table name starting with Lkp_
var match = sql.match(/(Lkp_[a-zA-Z0-9_]+)/i);

var lkp_table = "";
if (match != null && match.length > 1) {
    lkp_table = match[1];
}

// return the lookup table name
lkp_table;