使用Java中的正则表达式从字符串中提取双精度或整数？

Question

i want to extract an equation constants(ie integer or double NOT followed by a letter like say "x") from a string. eg eqn = "30.14x^2 + 55.69x + 60.1 = 100" . So far, i can get the coefficients of x using

([\\+\\-])?(\\d+(\\.\\d+)?)x

which gives 55.69x or 30.14x in this case.

Now i want to get the constants and I've tried regex below to extract +60.1 or 100

(?<!^)([\\+\\-])?(\\d+(\\.\\d+)?)(?!x)

However this doesn't work.Any help would be highly appreciated since I've on this for a month or so.

Answer 1

what about

 (?<!\^)(-|\+)?[\d.]++(?!x)

?

this variant uses the possessive quantifier ( the "++"), may be less strict with invalid numbers, but if the strings are syntactically correct, this does not matter.

Answer 2

This should work:

([\\+\\-])?(\\d+(\\.\\d+)?)(?![\\d.x])

[\\\\dx] works because it eliminates partial numbers. Eg: For 30.14x, 30.14 wouldn't match (?!x) but 30.1 would.

Answer 3

Here's a way to extract those values in a couple of passes:

public static void main(String[] args) {

    String eqn = "30.14x^2 + 55.69x + 60.1 = 100";

    String numberRegex = "[\\d]+(?:[\\.]{1}[\\d]+)?";
    String symbolsRegex = "=+-/\\\\*\\^";

    String coefRegex = "("+numberRegex+")([a-z]{1})";
    Pattern p = Pattern.compile(coefRegex);
    Matcher m = p.matcher(eqn);
    while (m.find()) {
        System.out.println("coefficient: " + m.group(1) + " -- " + m.group(2));
    }

    String constRegexp = "([^ " + symbolsRegex + "]" + numberRegex + "(?:[ ]{1}|$))";
    p = Pattern.compile(constRegexp);
    m = p.matcher(eqn);
    while (m.find()) {
        System.out.println("constant: " + m.group(1));
    }

}

Output:

coefficient: 30.14 -- x
coefficient: 55.69 -- x
constant: 60.1 
constant: 100

Just to add the literal values of the regex I used above:

String coefRegex = "([\\d]+(?:[\\.]{1}[\\d]+)?)([a-z]{1})";
String constRegexp = "([^ =+-/\\\\*\\^][\\d]+(?:[\\.]{1}[\\d]+)?(?:[ ]{1}|$))";

Answer 4

What is wrong with your regex is that you did not escape ^ , thus it means beginning of the string. And you also had not accounted for the substrings preceded by a digit or a decimal-point. Neither for the ones proceeded by one. Hence the correct form of your regex would be:

(?<!(\\^|\\d|\\.))[+-]?(\\d+(\\.\\d+)?)(?!(x|\\d|\\.))

which works ok. And also if you care about the exponent on x you might wanna go like this:

([+-])?(\\d+(\\.\\d+)?)x(^(\d+))?

by the way, you don't need to escape + inside [] , and - when it is next to [ of ] .