Can't undertand why my regex in Java doesn't work

Question

I'm trying to find pieces of text on the webpage I fetch that lay between 'align="left">\n" and '</form>\n</td>' substrings.

I wrote a regex:

(align=\"left\">\\n)(?<part>.*?)(<\/form>\\n<\/td>)

and tested it at https://www.freeformatter.com/java-regex-tester.html where it works just as I need.

But in the Java code it can't find anything.

My test code that I'm trying make working:

String frontPage = "<html>\n<head>\n<title>Hello</title>\n</head>\n" + 
            "<body>\n<table>\n<tr align=\"left\">\n" + 
            "<td>Hello \n<form>\n<input type=\"submit\" value=\"ok\">\n" + 
            "</form>\n</td>\n" + 
            "<td>World \n<form>\n<input type=\"submit\" value=\"ok\">\n" + 
            "</form>\n</td>\n" + 
            "</tr>\n</table>\n</body>\n</html>";

java.util.regex.Pattern p =
                java.util.regex.Pattern.compile(
                        "(align=\"left\">\\n)(?<part>.*?)(<\\/form>\\n<\\/td>)");
java.util.regex.Matcher m = p.matcher(frontPage);

List<String> parts = new ArrayList<>();
while (m.find()) {
    parts.add(m.group("part")); 
}
if (parts.size() == 0)
    System.out.println("No page parts found");
else {
    System.out.println("Something matches at least");
}

It finds matches if only first two groups specified, but when I add at least simple (form) sequence to the last group, it stops matching anything, and I can't even guess why.

related: https://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags — OH GOD SPIDERS, Aug 11 '17 at 09:35
@JFMeier I would rather ask why it ENDS with a question mark — Maurice Perry, Aug 11 '17 at 09:42
@JFMeier It defines group name so I don't need to think about group index. http://www.regular-expressions.info/refext.html — Dmitry Samoylov, Aug 11 '17 at 09:43
@MauricePerry It ends with a question mark to make * selector non-greedy. Otherwise it would return only one match instead of two matches I need. — Dmitry Samoylov, Aug 11 '17 at 09:44
your regexp searches for `\\n`, a literal `\ ` character followed by the character `n` but in the `frontPage` you have `\n`, a *new line* character. — Timothy Truckle, Aug 11 '17 at 09:49
@OHGODSPIDERS I guess it's a gentle hint about not using regular expressions there. I believe it's wrong since I need just `Hello` and `World` substrings from that HTML mass, nothing more. I don't think the task is complicated enough to add something like jsoup to the project. — Dmitry Samoylov, Aug 11 '17 at 09:51
@TimothyTruckle In a regex the literal `\n` matches `newline`. — SamWhan, Aug 11 '17 at 09:51
@ClasG yes, and that seams to be the OPs intention but the regexp is wrong for that. — Timothy Truckle, Aug 11 '17 at 09:53

SamWhan · Accepted Answer · 2017-08-11T10:20:47.407

4

Add DOTALL to the compile. Like

java.util.regex.Pattern.compile(
        "(align=\"left\">\\n)(?<part>.*?)(<\\/form>\\n<\\/td>)",
        java.util.regex.Pattern.DOTALL
);

See it here at ideone.

edited Aug 11 '17 at 10:20

answered Aug 11 '17 at 09:48

SamWhan

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Can't undertand why my regex in Java doesn't work

1 Answers1