使用正则表达式提取特定模式

Question

I'm having a hard time using regular expressions in Java even after reading numerous tutorials online. I'm trying to extract parts of a String received to be used later in my application.

Here are examples of the possible String received:

53248 <CERCLE> 321 211 55 </CERCLE>
57346 <RECTANGLE> 272 99 289 186 </RECTANGLE>

The first number is to be extracted as a sequence number. The word between <> is to be extracted as well. Then, the sequence of numbers in between as well.

Here is my pattern:

"(\\d+)\\s*<(\\w+)>\\s*((\\d+\\s*)+)\\s*</\\w*>.*"

Here is the code for my method so far:

public decompose(String s) throws IllegalArgumentException {

    Pattern pattern = Pattern.compile(PATTERN);
    Matcher matcher = pattern.matcher(s);

    noSeq = Integer.parseInt(matcher.group(1));
    type = typesFormes.valueOf(matcher.group(2));
    strCoords = matcher.group(3).split(" ");

}

Problem is that when I run the code, all my matcher groups are at -1 for some reason (not found I guess). I've been banging my head on this for a while and any suggestion is welcome :) Thanks.

Answer 1

Simply try with String#split()

  String str="53248 <CERCLE> 321 211 55 </CERCLE>";
  String[] array=str.split("(\\s<|>\\s)"); 
  // simple regex (space < OR > space)

Note: Try with \\\\s+ if there are one ore more spaces.

Use first three values of array that will be 53248, CERCLE, 321 211 55 in this case.

---

Complete code:

String str = "53248 <CERCLE> 321 211 55 </CERCLE>";
String[] array = str.split("(\\s<|>\\s)");

int noSeq = Integer.valueOf(array[0]);
String type = array[1];
String strCoords = array[2];

System.out.println(noSeq+", "+type+", "+strCoords);

output:

53248, CERCLE, 321 211 55

Answer 2

You just needed to tell the matcher to start matching the pattern against the input string. This works for me on ideone :

String s = "53248 <CERCLE> 321 211 55 </CERCLE>";
String PATTERN = "(\\d+)\\s*<(\\w+)>\\s*((\\d+\\s*)+)\\s*</\\w*>.*";
Pattern pattern = Pattern.compile(PATTERN);
Matcher matcher = pattern.matcher(s);
matcher.find();                         // aye, there's the rub
System.out.println(matcher.group(1));
System.out.println(matcher.group(2));
System.out.println(matcher.group(3));

Output was:

53248
CERCLE
321 211 55

The find() method, when successful, will let the matcher yield the information you want. From the javadocs:

If the match succeeds then more information can be obtained via the start, end, and group methods.

group() says something similarly indicative, emphasis mine:

Returns the input subsequence captured by the given group during the previous match operation.

Answer 3

As @2rs2ts pointed out, the problem was the missing matcher.find() call.

I would further improve like this:

final String PATTERN = "(\\d+)\\s*<(\\w+)>\\s*([\\d\\s]+)\\s*</\\2>.*";
String s = "53248 <CERCLE> 321 211 55 </CERCLE>";
Pattern pattern = Pattern.compile(PATTERN);
Matcher matcher = pattern.matcher(s);
if (matcher.find()) {
    System.out.println(matcher.group(1));
    System.out.println(matcher.group(2));
    System.out.println(matcher.group(3).trim());
}

Some improvements:

• In the pattern, you can simplify ((\\\\d+\\\\s*)+) as ([\\\\d\\\\s]+) . For your purpose, it's equivalent.
• In the pattern, you probably want to match <CERCLE> with a closing </CERCLE> , not </OTHER> . You can do that using \\\\2 , which is a back reference to the 2nd capture group.
• You can judge by the result of matcher.find() if anything was matched.
• Before you split the list of numbers in the middle, you might want to trim the possible trailing whitespace at the end using .trim() .