Matching one string multiple times using regex in Java

Question

I'm having some issues with making the following regex work. I would like the following string:

"Please enter your name here"

to result in an array with the following elements:

'please enter', 'enter your', 'your name', 'name here'

Currently, I'm using the following pattern, and then creating a matcher and iterating in the following way:

Pattern word = Pattern.compile("[\w]+ [\w]+");
Matcher m = word.matcher("Please enter your name here");

while (m.find()) {
    wordList.add(m.group());
}

But the result I'm getting is:

'please enter', 'your name'

What am I doing wrong? (P.s., i checked the same regex on regexpal.com and had the same problem). It seems like the same word won't be matched twice. What can I do to achieve the result I want?

Thanks.

---------------------------------

EDIT: Thanks for all the suggestions! I ended up doing this (because it adds flexibility in being able to easily specify number of "n-grams"):

Integer nGrams = 2;
String patternTpl = "\\b[\\w']+\\b";
String concatString = "what is your age? please enter your name."
for (int i = 0; i < nGrams; i++) {
    // Create pattern.
    String pattern = patternTpl;
    for (int j = 0; j < i; j++) {
        pattern = pattern + " " + patternTpl;
    }
    pattern = "(?=(" + pattern + "))";
    Pattern word = Pattern.compile(pattern);
    Matcher m = word.matcher(concatString);

    // Iterate over all words and populate wordList
    while (m.find()) {
        wordList.add(m.group(1));
    }
}

This results in:

Pattern: 
(?=(\b[\w']+\b)) // In the first iteration
(?=(\b[\w']+\b \b[\w']+\b)) // In the second iteration

Array:
[what, is, your, age, please, enter, your, name, what is, is your, your age, please enter, enter your, your name]

Note: Got the pattern from the following top answer: Java regex skipping matches

split the string with space, you got words array, you can get element[i] and element[i+1]. of course be careful about the OutOfBoundEx — Kent
– Kent, Commented Sep 11, 2013 at 21:25

arshajii · Accepted Answer · 2013-09-11 21:24:43Z

9

The matches can't overlap, which explains your result. Here's a potential workaround, making use of capturing groups with a positive lookahead:

Pattern word = Pattern.compile("(\\w+)(?=(\\s\\w+))");
Matcher m = word.matcher("Please enter your name here");

while (m.find()) {
    System.out.println(m.group(1) + m.group(2));
}

Please enter
enter your
your name
name here

answered Sep 11, 2013 at 21:24

arshajii

130k26 gold badges246 silver badges293 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

foglerek Over a year ago

Thanks, this was the closest to what I wanted to do, and this might be more efficient than the edit I did in the end. I'll look into perhaps using this instead of what I ended up doing.

foglerek Over a year ago

How would I make this work with any number of "n-grams"? So if I would also like to match "Please enter your", "enter your name", "your name here"? EDIT: Figured it out, I can just add more (?=(\\s\\w+)), based on the number of n-grams needed.

Josh M · Accepted Answer · 2013-09-11 21:33:01Z

1

If you want to avoid using such specific RegEx, perhaps you should try a simpler, and more easier, solution:

public static String[] array(final String string){
    final String[] words = string.split(" ");
    final String[] array = new String[words.length-1];
    for(int i = 0; i < words.length-1; i++)
        array[i] = String.format("%s %s", words[i], words[i+1]);
    return array;
}

public static void main(String args[]){
    final String[] array = array("Please enter your name here");
    System.out.println(Arrays.toString(array));
}

The output is:

[Please enter, enter your, your name, name here]

answered Sep 11, 2013 at 21:33

Josh M

12k8 gold badges42 silver badges49 bronze badges

1 Comment

foglerek Over a year ago

I might look into using this solution as well, it certainly is more efficient than looping through all the words as I'm doing now for nGrams > 1.

willeM_ Van Onsem · Accepted Answer · 2013-09-11 21:24:25Z

0

You're not doing anything wrong. It's just the way a regex works (otherwise matching would become O(n^2), since regex matching is done in linear time, this cannot be processed).

In this case you could simply search for [\w]+. And postprocess these groups.

answered Sep 11, 2013 at 21:24

willeM_ Van Onsem

482k33 gold badges483 silver badges624 bronze badges

Comments

ajb · Accepted Answer · 2013-09-11 21:31:36Z

0

Something like:

Pattern word = Pattern.compile("(\\w+) ?");
Matcher m = word.matcher("Please enter your name here");

String previous = null;
while (m.find()) {
    if (previous != null)
        wordList.add(previous + m.group(1));
    previous = m.group();
}

The pattern ends with an optional space (which matches if there are more spaces in the string). m.group() returns the entire match, with the space; m.group(1) returns just the word, without the space.

answered Sep 11, 2013 at 21:31

ajb

31.8k4 gold badges63 silver badges86 bronze badges

Collectives™ on Stack Overflow

Matching one string multiple times using regex in Java

4 Answers 4

2 Comments

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related