This grammar will solve the problem in ANTLR4:
lexer grammar TextLexer;
COMMENT_BEGIN: 'comment' -> more,pushMode(MCOMMENT);
WORD_BEGIN: [a-z] -> more, pushMode(MWORD);
WS: ' ' -> channel(HIDDEN);
mode MCOMMENT;
COMMENT: .+? 'end'-> mode(DEFAULT_MODE);
mode MWORD;
WORD: [a-z]+ -> mode(DEFAULT_MODE);