C#

^\d+$

^[0-9]

if(s.Length == 0) return neither;
else if(s[0] == '-') return negative;
else if(s[0] >= '0' && s[0] <= '9') return positive;
else return non-numeric;

List<SalesTeam> teamNorth;
List<SalesTeam> teamSouth;
List<SalesTeam> teamEast;

class SalesTeam {
	public string name;
}
class SalesTeamNorth : SalesTeam {
	public SalesTeamNorth(int v) { name="N"+v; }
}
class SalesTeamSouth : SalesTeam {
	public SalesTeamSouth(int v) { name="S"+v; }
}
public override void Test(int arg) {
	List<SalesTeam> teamsNorth=new List<SalesTeam>();
	for(int i=0; i<3; i++) teamsNorth.Add(new SalesTeamNorth(i));
	List<SalesTeam> teamsSouth=new List<SalesTeam>();
	for (int i=0; i<3; i++) teamsSouth.Add(new SalesTeamSouth(i));
	
	// now iterate all teams
	foreach (List<SalesTeam> teams in new List<List<SalesTeam>>(){ teamsNorth, teamsSouth }) {
		foreach (SalesTeam team in teams) {
			log(team.name);
		}
	}
}

IList<AllSalesReps> allReps = teamNorth
    .Cast<AllSalesReps>()
    .Concat(teamSouth)
    .Concat(teamEast)
    .ToList();