How to convert python regex to java regex?

Question

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?

"but it is still not matching the way I want" can we see how exactly you are using this regex in Java? What is your input, expected and actual result? — Pshemo
– Pshemo, Commented Apr 17, 2017 at 16:01
Guess: You're using matches(). Don't, because that always matches against the entire input. Use find(). — Andreas
– Andreas, Commented Apr 17, 2017 at 16:11
Yes, I mean in Java. That is what you're asking, right? How to do it in Java? And my guess is that you're using the wrong Java method. — Andreas
– Andreas, Commented Apr 17, 2017 at 16:14

Andreas · Accepted Answer · 2017-04-17 20:37:32Z

2

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

edited Apr 17, 2017 at 20:37

answered Apr 17, 2017 at 20:29

Andreas

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2 Comments

makansij Over a year ago

I'm not of age to vote, but +1 @Andreas! Is this true for scala as well?

Andreas Over a year ago

@jaja You should still be able to accept the answer by clicking the checkmark. Sorry, I don't know Scala.

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