Setting up a Regex Pattern in Java

Question

I am having some difficulty trying to create a working regex pattern for recognizing a string of file names.

Pattern pattern = Pattern.compile("(\\w+)(\\.)(t)(x)(t)(\\s$|\\,)");

When using a .find() from a matcher class on my sample input, say

"file1.txt,file2.txt "

I am returned true, which is fine, however other erroneous input also returns true.

This erroneous input includes strings such as:

"file1.txt,,file2.txt "

"file%.text "

I have been consulting this website as I have been trying to construct them, I'm pretty sure I am missing something rather obvious though. Link

"I'm not quite getting the results I would like." which are...? — user1803551
– user1803551, Commented Sep 19, 2014 at 13:16
@user1803551, I should have explained myself a bit more for the first input listed, I would like it to return true, for the two secondary examples, those should return false. — JVB
– JVB, Commented Sep 19, 2014 at 13:17
I guess you are trying to get .txt extensions, one at least, and the others separated by commas. Try this ([A-Za-z0-9]+\\.txt)+(,[A-Za-z0-9]+\\.txt)* — Luke SpringWalker
– Luke SpringWalker, Commented Sep 19, 2014 at 13:17
Try an online regex checker... to learn how to get it right... — vikingsteve
– vikingsteve, Commented Sep 19, 2014 at 13:18

Mena · Accepted Answer · 2014-09-19 13:22:13Z

To validate your file name list, you can use the following solution:

//                                  | preceded by start of input, or
//                                  |    | comma, preceded itself by either word or space
//                                  |    |              | file name
//                                  |    |              |     | dot
//                                  |    |              |     |    | extension
//                                  |    |              |     |    |  | optional 1 space
//                                  |    |              |     |    |  |   | followed by end of input
//                                  |    |              |     |    |  |   |     | or comma, 
//                                  |    |              |     |    |  |   |     | followed itself by word character
Pattern pattern = Pattern.compile("(?<=^|(?<=[\\w\\s]),)(\\w+)(\\.)txt\\s?(?=$|,(?=\\w))");
String input = "file1.txt,file2.txt";
String badInput = "file3.txt,,file4.txt";
String otherBadInput = "file%.txt, file!.txt";
Matcher m = pattern.matcher(input);
while (m.find()) {
    // printing group 1: file name
    System.out.println(m.group(1));
}
m = pattern.matcher(badInput);
// won't find anything
while (m.find()) {
    // printing group 1: file name
    System.out.println(m.group(1));
}
m = pattern.matcher(otherBadInput);
// won't find anything
while (m.find()) {
    // printing group 1: file name
    System.out.println(m.group(1));
}

Output

file1
file2

Nicolas Albert · Accepted Answer · 2014-09-19 13:35:42Z

2

May this help you:

Pattern pattern = Pattern.compile("(.*?\\.\\w+)(?:,|$)");
String files = "file1.txt,file2.txt";
Matcher mFile = pattern.matcher(files);
while (mFile.find()) {
    System.out.println("file: " + mFile.group(1));
}

Output:

file: file1.txt
file: file2.txt

With only .txt files: (.*?\\.txt)(?:,|$)

edited Sep 19, 2014 at 13:35

answered Sep 19, 2014 at 13:19

Nicolas Albert

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1 Comment

JVB Over a year ago

This expression gave me the results I was looking for, thank you very much @Nicolas.

NoobEditor · Accepted Answer · 2014-09-19 13:28:11Z

1

Naive way :

    public boolean validateFileName(String string){
      String[] fileNames= string.split(",");
      Pattern pattern = Pattern.compile("\b[\w]*[.](txt)\b"); /*match complete name, not just pattern*/
      for(int i=0;i<fileNames.length;i++){
          Matcher m = p.matcher(fileNames[i]);
          if (!m.matches())
              return false;
      }
      return true;
    }

edited Sep 19, 2014 at 13:28

answered Sep 19, 2014 at 13:23

NoobEditor

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Comments

iGeeks · Accepted Answer · 2014-09-19 13:39:00Z

1

Pattern p = Pattern.compile("((\\w+\\.txt)(,??|$))+");

The above Pattern lets "file1.txt,,file2.txt" pass, but does not get an empty file for the two commas. The other Strings "file1.txt,file2.txt" and "file%txt" are processed correctly.

answered Sep 19, 2014 at 13:39

iGeeks

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