Java regex grouping

Question

I have the following entry in a properties file:

some.key = \n
  [1:Some value] \n
  [14:Some other value] \n
  [834:Yet another value] \n

I am trying to parse it using a regular expression, but I can't seem to get the grouping correct. I am trying to print out a key/value for each entry. Example: Key="834", Value="Yet another value"

private static final String REGEX_PATTERN = "[(\\d+)\\:(\\w+(\\s)*)]+";

private void foo(String propValue){
    final Pattern p = Pattern.compile(REGEX_PATTERN);
    final Matcher m = p.matcher(propValue);
    while (m.find()) {
        final String key = m.group(0).trim();
        final String value = m.group(1).trim();
        System.out.println(String.format("Key[%s] Value[%s]", key, value));            
    }
}

The error I get is:

Exception: java.lang.IndexOutOfBoundsException: No group 1

I thought I was grouping correctly in the regex but I guess not. Any help would be appreciated!

Thanks

UPDATE: Escaping the brackets worked. Changed the pattern to the followingThanks for the feedback!

 private static final String REGEX_PATTERN = "\\[(\\d+)\\:(\\w+(\\w|\\s)*)\\]+";

Index out of bounds: checking for an element in the array that does not exist. Probably pointed to an unsetted index — Alfabravo
– Alfabravo, Commented May 17, 2012 at 13:18
Alfabravo - Yes, I am aware, but I am curious why the regular expression is incorrect — user973479
– user973479, Commented May 17, 2012 at 13:20

jpaugh · Accepted Answer · 2012-05-17 13:20:28Z

2

[ should be escaped (as well as ]).

"\\[(\\d+)....\\]+"

[] Is used for character classes: [0-9] == (0|1|2|...|9)

answered May 17, 2012 at 13:20

jpaugh

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Comments

Dan Manastireanu · Accepted Answer · 2012-05-17 13:31:13Z

2

Try this:

private static final String REGEX_PATTERN = "\\[(\\d+):([\\w\\s]+)\\]";

final Pattern p = Pattern.compile(REGEX_PATTERN);
final Matcher m = p.matcher(propValue);
while (m.find()) {
    final String key = m.group(1).trim();
    final String value = m.group(2).trim();
    System.out.println(String.format("Key[%s] Value[%s]", key, value));
}

the [ and ] need to be escaped because they represent the start and end of a character class
group(0) is always the full match, so your groups should start with 1
note how I wrote the second group [\\w\\s]+. This means a character class of word or whitespace characters

answered May 17, 2012 at 13:31

Dan Manastireanu

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Comments

mathematical.coffee · Accepted Answer · 2012-05-17 13:20:31Z

1

It's your regex, [] are special characters and need to be escaped if you want to interpret them literally.

Try

"\\[(\\d+)\\:(\\w+(\\s)*)\\]"

Note - I removed the '+'. The matcher will keep finding substrings that match the pattern so the + is not necessary. (You might need to feed in a GLOBAL switch - I can't remember).

I can't help but feel this might be simpler without regex though, perhaps by splitting on \n or [ and then splitting on : for each of those.

answered May 17, 2012 at 13:20

mathematical.coffee

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AlexR · Accepted Answer · 2012-05-17 13:20:03Z

0

Since you are using string that consists of several lines you should tell it to Pattern:

final Pattern p = Pattern.compile(REGEX_PATTERN, Pattern.MULTILINE);

Although it is irrelevant directly for you I'd recommend you to add DOTALL too:

final Pattern p = Pattern.compile(REGEX_PATTERN, Pattern.MULTILINE | Pattern.DOTALL);

answered May 17, 2012 at 13:20

AlexR

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1 Comment

jpaugh Over a year ago

The multiline thing is only important if you want to detect start/end of line. Probably a good idea to do so, though.

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