正则表达式验证数字0-100，最多两位小数

Question

So I know it would be easier to just use the php is_numeric function, but what i'm trying to do is create a regular expression, mostly for learning regex, to test test scores. The range can be 0-100, but can have 2 decimal places. This is what I have so far and its not working.

  if (preg_match("^([0-9]){2,3})(.[0-9]{2})?$([0-9]{1})?$", $quiz1))
    {
        echo "match";
    }
    else {
        $error = true;
    }

If I'm thinking correctly the literal meaning:

start of string find characters 0-9, for two places. optional end of string decimal and find characters 0-9, for two places. optional end of string find characters 0-9, for 1 place.

Answer 1

Why not something like this?

 /^(?:100|\d{1,2})(?:\.\d{1,2})?$/

• ^ - beginning of string
• (?:100|\\d{1,2}) - non-capturing group, 100 or 0-99
• (?:.\\d{1,2})? - optional non-capturing group (.# or .##)
• $ - end of string

Results:

php > $tests = array(0, 5, 10, 50, 100, 99.5, 75.43, 75.436, 101);

php > foreach ($tests as $test) { print $test . " - " . preg_match("/^(?:100|\\d{1,2})(?:\\.\\d{1,2})?$/", $test) . "\\n"; }

0 - 1

5 - 1

10 - 1

50 - 1

100 - 1

99.5 - 1

75.43 - 1

75.436 - 0

101 - 0

75F43 - 0

Yours doesn't work even when I add the slashes and remove the extra ) .

php > foreach ($tests as $test) { print $test . " - " . preg_match("/^([0-9]{2,3})(.[0-9]{2})?$([0-9]{1})?$/", $test) . "\\n"; } 0 - 0

5 - 0

10 - 1

50 - 1

100 - 1

99.5 - 0

75.43 - 1

75.436 - 0

101 - 1

75F43 - 1

Answer 2

Surround your expression with / characters

http://php.net/manual/en/regexp.reference.delimiters.php

Answer 3

gpojd's answer is correct. However, here's why your regex isn't working.

First of all, you want the first $ inside the ()'s. Because otherwise it will need to match the end of the string, and then later after that the end of the string again, which is of course impossible.

Second, the dot character needs to become \\. because a dot by itself matches any character, but you want an actual dot. Lastly, you need delimiters, like someone else suggested. Here's what your regex matches:

first, two or three digits, mandatory. Then, any character followed by two digits, optionally. Then, the end of the string, mandatory. Then, one digit, optionally. Then, the end of the string again, mandatory.

Since you're a programmer, it can be hard to get used to the way of thinking with regular expressions; you're used to thinking in terms of if-else statements but regular expressions don't really work that way.

Again, the option above is good enough but if I were to write a regex for this I'd put:

/^100(.00)?|([1-9]?[0-9])(\.[0-9]{2})?$/

So, either 100 followed by an optional .00, or: first, an optional nonzero digit and then a mandatory digit (those make the numbers 0 to 99), then optionally a dot followed by two digits.