如何将字符串中的数学表达式转换为整数 [英] how to translate math espression in a string to integer

问题描述

例如，我有一条声明:

$var = '2*2-3+8'; //variable type is string

如何使其等于9?

推荐答案

来自 Richard van Velzen 编写.一旦有了它，并且它匹配，就可以放心，可以在字符串上使用eval了.使用eval之前，请务必确保输入已通过验证！

From this page, a very awesome (simple) calculation validation regular expression, written by Richard van Velzen. Once you have that, and it matches, you can rest assured that you can use eval over the string. Always make sure the input is validated before using eval!

<?php
$regex = '{
    \A        # the absolute beginning of the string
    \h*        # optional horizontal whitespace
    (        # start of group 1 (this is called recursively)
    (?:
        \(        # literal (

        \h*
        [-+]?        # optionally prefixed by + or -
        \h*

        # A number
        (?: \d* \. \d+ | \d+ \. \d* | \d+) (?: [eE] [+-]? \d+ )?

        (?:
            \h*
            [-+*/]        # an operator
            \h*
            (?1)        # recursive call to the first pattern.
        )?

        \h*
        \)        # closing )

        |        # or: just one number

        \h*
        [-+]?
        \h*

        (?: \d* \. \d+ | \d+ \. \d* | \d+) (?: [eE] [+-]? \d+ )?
    )

    # and the rest, of course.
    (?:
        \h*
        [-+*/]
        \h*
        (?1)
    )?
    )
    \h*

    \z        # the absolute ending of the string.
}x';

$var = '2*2-3+8';

if( 0 !== preg_match( $regex, $var ) ) {
    $answer = eval( 'return ' . $var . ';' );
    echo $answer;
}
else {
    echo "Invalid calculation.";
}

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