浮点数可以由以下的正则表达式确定：
white_space_opt [[:space:]]*
digit_sequence [[:digit:]]+
fractional_constant {digit_sequence}\.?|\.{digit_sequence}|{digit_sequence}\.{digit_sequence}
exponent_part [eE][+-]?{digit_sequence}
floating_constant {fractional_constant}{exponent_part}*
accepted_input {white_space_opt}[+-]?{floating_constant}{white_space_opt}
---------------------------------------------
如果不接受 1 2 3 这样的数为浮点数， 则fractional_constant应该是 {digit_sequence}\.|.....
如果不接受 浮点数两端加的空格,则 accepted_input 应该是 [+-]?{floating_constant}
---------------------------------------------
你可以用你稀饭的正则表达式库，如posix regex , pcre , boost::regex 啥的做这件事，也可以用其对应的 DFA 做判断， 比如这样
#include <stdio.h>
int
is_floating_constant(
const
char
* input )
{
static
const
int
yy_accept[21] =
{ 0,
0, 0, 3, 2, 2, 2, 2, 1, 0, 0,
0, 1, 1, 1, 1, 0, 1, 0, 1, 0
} ;
static
const
int
yy_ec[256] =
{ 0,
1, 1, 1, 1, 1, 1, 1, 1, 2, 2,
2, 2, 2, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 2, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 3, 1, 3, 4, 1, 5, 5, 5,
5, 5, 5, 5, 5, 5, 5, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 6, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
6, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1
} ;
static
const
int
yy_meta[7] =
{ 0,
1, 2, 1, 1, 3, 1
} ;
static
const
int
yy_base[24] =
{ 0,
0, 0, 27, 28, 5, 7, 0, 11, 0, 0,
0, 0, 16, 0, 21, 20, 0, 0, 19, 28,
17, 17, 11
} ;
static
const
int
yy_def[24] =
{ 0,
20, 1, 20, 20, 20, 20, 21, 20, 5, 6,
21, 8, 20, 22, 13, 20, 15, 23, 15, 0,
20, 20, 20
} ;
static
const
int
yy_nxt[35] =
{ 0,
4, 5, 6, 7, 8, 4, 9, 10, 11, 12,
11, 12, 14, 19, 15, 12, 16, 14, 14, 13,
13, 16, 18, 19, 19, 17, 20, 3, 20, 20,
20, 20, 20, 20
} ;
static
const
int
yy_chk[35] =
{ 0,
1, 1, 1, 1, 1, 1, 5, 5, 5, 5,
6, 6, 8, 23, 8, 8, 8, 13, 22, 21,
13, 13, 16, 19, 16, 15, 3, 20, 20, 20,
20, 20, 20, 20
} ;
enum
{ last_dfa = 19 , jambase = 28 };
int
curr = 1 , c ;
do
{
if
( 0 == *input )
break
;
c = yy_ec[*(unsigned
char
*)input];
if
( 1 == yy_accept[curr] && 0 == input[1] )
return
1;
while
( yy_chk[ yy_base[curr] + c ] != curr )
if
( (curr=yy_def[curr] ) >= last_dfa + 2 )
c = yy_meta[c];
curr = yy_nxt[ yy_base[curr] + c ];
++input;
}
while
( yy_base[curr] != jambase );
return
0;
}
int
main()
{
char
input[80+1] =
""
;
while
(
fgets
( input , 80 , stdin ) )
{
if
( is_floating_constant( input ) )
printf
(
"YES\n"
);
else
printf
(
"NO\n"
);
}
return
0;
}
///////////////////////////////////////////////////////////////////////
唉，又错了，应该是下面的。。。
floating_constant {fractional_constant}{exponent_part}？
对应的DFA表也要改为：
static
const
int
yy_accept[21] =
{ 0,
0, 0, 3, 2, 2, 2, 2, 1, 0, 0,
0, 1, 1, 1, 1, 0, 1, 0, 1, 0
} ;
static
const
int
yy_ec[256] =
{ 0,
1, 1, 1, 1, 1, 1, 1, 1, 2, 2,
2, 2, 2, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 2, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 3, 1, 3, 4, 1, 5, 5, 5,
5, 5, 5, 5, 5, 5, 5, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 6, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
6, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1
} ;
static
const
int
yy_meta[7] =
{ 0,
1, 2, 1, 1, 3, 1
} ;
static
const
int
yy_base[24] =
{ 0,
0, 0, 27, 28, 5, 7, 0, 11, 0, 0,
0, 0, 16, 0, 21, 20, 0, 0, 22, 28,
17, 17, 11
} ;
static
const
int
yy_def[24] =
{ 0,
20, 1, 20, 20, 20, 20, 21, 20, 5, 6,
21, 8, 20, 22, 13, 20, 15, 23, 23, 0,
20, 20, 20
} ;
static
const
int
yy_nxt[35] =
{ 0,
4, 5, 6, 7, 8, 4, 9, 10, 11, 12,
11, 12, 14, 19, 15, 12, 16, 14, 14, 13,
13, 16, 18, 14, 19, 17, 20, 3, 20, 20,
20, 20, 20, 20
} ;
static
const
int
yy_chk[35] =
{ 0,
1, 1, 1, 1, 1, 1, 5, 5, 5, 5,
6, 6, 8, 23, 8, 8, 8, 13, 22, 21,
13, 13, 16, 19, 16, 15, 3, 20, 20, 20,
20, 20, 20, 20
} ;
enum
{ last_dfa = 19 , jambase = 28 };