Python和Java中相同正则表达式的不同行为

Question

Firstly, my apologies as I don't know regular expressions that well.

I am using a regular expression to match a string. I tested it in Python command line interface, but when I ran it in Java, it produced a different result.

Python execution:

re.search("[0-9]*[\\.[0-9]+]?[^0-9]*D\\([M|W]\\)\\s*US", "9.5 D(M) US");

gives the result as:

<_sre.SRE_Match object; span=(0, 11), match='9.5 D(M) US'>

But the Java code

import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

class RegexTest {
    private static final Pattern FALLBACK_MEN_SIZE_PATTERN = Pattern.compile("[0-9]*[\\.[0-9]+]?[^0-9]*D\\([M|W]\\)\\s*US");

    public static void main(String[] args) {
    String strTest = "9.5 D(M) US";
    Matcher matcher = FALLBACK_MEN_SIZE_PATTERN.matcher(strTest);
        if (matcher.find()) {
            System.out.println(matcher.group(0));
        }
    }
}

gives the output as:

5 D(M) US

I don't understand why it is behaving the different way.

Answer 1

Here is the pattern that will work the same in Java and Python:

"[0-9]*(?:\\.[0-9]+)?[^0-9]*D\\([MW]\\)\\s*US"

See Python and Java demos.

In Python, [\\\\.[0-9]+]? is read as 2 subpatterns: [\\.[0-9]+ (1 or more . s, [ s, or digits) and ]? (0 or 1 ] ). See how your regex works in Python here . Or, with more detalization with capturing groups, here .

In Java, it is read as one single character class (ie [ and ] inside are ignored as they cannot be parsed correctly by the regex engine, thus the whole subpattern standing for 0 or 1 . , a digit, or + ) and since it is optional, it was not capturing anything (you can get a visual hint at Visual Regex Tester , type 123.+[] as input and [\\.[0-9]+]? as regex).

And a final touch: [M|W] stands for M , | , or W , while I think you meant [MW] = M or W .

Answer 2

I'm not a Python expert, so I can't tell you why it worked on Python, but in Java, your problem is the [\\\\.[0-9]+]? part. You probably meant it to be (\\\\.[0-9]+)? .

As it is, it's a list of characters inside a [] followed by a ? . That is, this part of the expression only matches a single or zero character, so it cannot match the .5 together.

Here is an illustration of the matching attempts:

Now, if your pattern used () instead of [] , this would be the result: