how to compare two string with a specific number forme ?

I fix it like this

 

foreach (string n in ls_Nfacture)
{
string a= n.TrimStart(new char[] {' '});
string b = a.TrimStart(new char[] { 'F', 'A', 'C', 'T', 'U', 'R', 'E'});
string D= b.TrimEnd(new char[] {' '});

ls_num_Fact.Add(D);
}