编写如下 pascal 子集的词法分析程序

以下是一个简单的基于Pascal子集的词法分析程序：

program Lexer;

const
  MAXLEN = 100;

type
  TokenType = (ID, NUM, PLUS, MINUS, MULT, DIV, LPAREN, RPAREN, ASSIGN, SEMI, ERROR, EOF);
  // 定义Token类型

var
  TokenString: string;
  Token: TokenType;
  Line: string;
  LineIndex: integer;

function GetChar: char;
begin
  if LineIndex <= Length(Line) then
  begin
    GetChar := Line[LineIndex];
    LineIndex := LineIndex + 1;
  end
  else
    GetChar := #0;
end;

procedure GetToken;
var
  Ch: char;
begin
  TokenString := '';
  Token := ERROR;
  while (Token = ERROR) and (LineIndex <= Length(Line)) do
  begin
    Ch := GetChar;
    if Ch in ['a'..'z', 'A'..'Z'] then
    begin
      Token := ID;
      while Ch in ['a'..'z', 'A'..'Z', '0'..'9'] do
      begin
        TokenString := TokenString + Ch;
        Ch := GetChar;
      end;
      LineIndex := LineIndex - 1;
    end
    else if Ch in ['0'..'9'] then
    begin
      Token := NUM;
      while Ch in ['0'..'9'] do
      begin
        TokenString := TokenString + Ch;
        Ch := GetChar;
      end;
      LineIndex := LineIndex - 1;
    end
    else if Ch = '+' then
      Token := PLUS
    else if Ch = '-' then
      Token := MINUS
    else if Ch = '*' then
      Token := MULT
    else if Ch = '/' then
      Token := DIV
    else if Ch = '(' then
      Token := LPAREN
    else if Ch = ')' then
      Token := RPAREN
    else if Ch = '=' then
      Token := ASSIGN
    else if Ch = ';' then
      Token := SEMI;
  end;
end;

begin
  Line := 'a := 2 + 3;';
  LineIndex := 1;

  repeat
    GetToken;
    writeln(TokenString);
  until Token = EOF;
end.

以上代码实现了一个简单的词法分析程序，可以识别出变量名、数字、加减乘除、括号、赋值符号和分号。你可以根据需要修改 `Line` 的值来测试不同的输入。