PT[101]% ls
ch3-01.l	ch3-01.txt	ch3-01.y	ch3-02.y

PT[102]% bison -y -d ch3-01.y
PT[103]% ls
ch3-01.l	ch3-01.y	y.tab.c
ch3-01.txt	ch3-02.y	y.tab.h

PT[104]% flex ch3-01.l
PT[105]% gcc y.tab.c lex.yy.c -ll -ly

PT[106]% ./a.out
3 + 4
= 7
PT[107]% ./a.out
5 - 4 - 3
= -2
PT[108]% ./a.out
3 * 4
= 3
syntax error
PT[109]%