Delphi: A simple Like function

A simple Like function

Sometimes we need to know if a string matches a pattern, which is a string with wildcards (for example ? and *). For this purpose you can use the MatchesMask function that is declared in the Masks unit, but here as a teaching example and because we believe it might be useful in certain cases (for example when you don't want sets), we implement a replacement function that returns True if the string matches the pattern and False if not. Here is the source code:

uses SysUtils;

function Like(AString, Pattern: string): boolean;
var
  i, n, n1, n2: integer;
  p1, p2: pchar;
label
  match, nomatch;
begin
  AString := UpperCase(AString);
  Pattern := UpperCase(Pattern);
  n1 := Length(AString);
  n2 := Length(Pattern);
  if n1 < n2 then n := n1 else n := n2;
  p1 := pchar(AString);
  p2 := pchar(Pattern);
  for i := 1 to n do begin
    if p2^ = '*' then goto match;
    if (p2^ <> '?') and (p2^ <> p1^) then goto nomatch;
    inc(p1); inc(p2);
  end;
  if n1 > n2 then begin
nomatch:
    Result := False;
    exit;
  end else if n1 < n2 then begin
    for i := n1 + 1 to n2 do begin
      if not (p2^ in ['*','?']) then goto nomatch;
      inc(p2);
    end;
  end;
match:
  Result := True;
end;

Sample call

if Like('Walter', 'WA?T*') then
  ShowMessage('It worked!');

If you want to see another example, we use this function to determine if a file name matches a specification.