CODEWITHSUNDEEP
javapythonregexregex-negation
makansij
makansij

Reputation: 9875

How to convert python regex to java regex?

I have some regular expressions in Python which I need to convert to java. I know what I want the regular expression to do, but I just don't know how to convert it.

Here is the expression in python: ^172\.(1[6789]|2\d|30|31)\.. I want it to capture any sort of ip address like 172.X where X ranges from 16 to 31.

This works in python:

import re
pattern='^172\\.(1[6789]|2\\d|30|31)\\.'
test_strings = ['172.19.0.0', '172.24.0.0', '172.45.0.0', '172.19.98.94']
for string in test_strings:
    print re.findall(pattern, string)

and it appropriately captures what I expect:

['19']
['24']
[]
['19']

But I tried to convert these to java and itdoesn't work. It seems like I should be able to convert to a java regex simply by adding a \ to each \ to escape correctly? like ^172\\.(1[6789]|2\\d|30|31)\\.

but it is still not matching the way I want. what am I missing about the differences between python and JAVA regular expression in this case?

I do not have the java code easily available, but I tried this tool: http://java-regex-tester.appspot.com/, and I set the Target Text to 172.19.0.0 and it doesn't match, but it does "Find". However, when I input "blah" as the Target Text it ALSO puts something in the "Find" section...so I'm not sure I trust this tool http://java-regex-tester.appspot.com/ because it puts any string in "find", even when it is "blah".

So, how do I verify that my java regular expression is correct?

Upvotes: 0

Views: 2559

Answers (1)

Andreas
Andreas

Reputation: 159135

Java 8 does not have the equivalent of findall(), so you need to write your own find() loop and gather the results into a List, like this:

Pattern pattern = Pattern.compile("^172\\.(1[6789]|2\\d|30|31)\\.");
String[] test_strings = {"172.19.0.0", "172.24.0.0", "172.45.0.0", "172.19.98.94"};
for (String string : test_strings) {
    List<String> list = new ArrayList<>();
    for (Matcher matcher = pattern.matcher(string); matcher.find(); )
        list.add(matcher.group(1));
    System.out.println(list);
}

Output

[19]
[24]
[]
[19]

Or course, since your regex can find at most one match, your code should really be:

Pattern pattern = Pattern.compile("^172\\.(1[6789]|2\\d|30|31)\\.");
String[] test_strings = {"172.19.0.0", "172.24.0.0", "172.45.0.0", "172.19.98.94"};
for (String string : test_strings) {
    Matcher matcher = pattern.matcher(string);
    if (matcher.find())
        System.out.println(matcher.group(1));
    else
        System.out.println();
}

Output

19
24

19

Upvotes: 2

Related Questions