Regex for allowing the file extension only once

Question

My validation require a regex with following requirement:

The file extension should appear only once e.g.
- myfile.pdf(valid)
- myfile.pdf.pdf(invalid)
Valid file extensions are pdf, doc, docx:
- myfile.pdf(valid)
- myfile.doc(valid)
- myfile.txt(invalid)

Could you please help me with the regex expression in Java.

Would something like myFile.txt.pdf match? It has two extensions but only one is valid. — Sweeper, Jul 14 '17 at 13:19
What have you tried? Where is that failing? We're not a code writing service, please read the [help] for guidance on asking questions. — Erwin Bolwidt, Jul 14 '17 at 13:19
Is `pdf.pdf` valid? I think first `pdf` is not an extension. — , Jul 14 '17 at 13:25
I've tried `([^\s]+(\.(?i)(pdf|doc|docx))$)` but it only validate 2) — rowen, Jul 14 '17 at 13:26
It's strange that you would need that: in any file, extension is only a part of file name, nothing more. And file extension by convention is the part after last dot in name. — M. Prokhorov, Jul 14 '17 at 13:35
Thanks @Sweeper. but the regex you posted some time ago is returning true for `myfile.pdf.pdf` as well. — rowen, Jul 14 '17 at 13:43
@user868888 Yeah I tested it on regex101.com using the pcre flavour and it worked fine. https://regex101.com/r/6AEuW9/1 Apparently in this case the tiny differences between pcre and java mattered :) — Sweeper, Jul 14 '17 at 13:46

score 0 · Answer 1 · answered Jul 14 '17 at 13:52

I would suggest to check the requirements separatedly. Example:

public static void main(String[] args) throws Exception {
    String [] fileNames = {"myfile.pdf","myfile.pdf.pdf","myfile.doc","myfile.txt","myfile.txt.doc"};         
    for(String fileName : fileNames){
        boolean valid = hasValidExtension(fileName) && noDuplicates(fileName);
        System.out.println(fileName+"\t" + (valid?"Valid":"Invalid"));
    }
}

public static boolean hasValidExtension(String fileName){
    String ext = fileName.split("\\.(?=[^\\.]+$)")[1];
    if(ext.equals("pdf")||ext.equals("doc")||ext.equals("docx")){
        return true;
    }
    return false;
}

public static boolean noDuplicates(String fileName){
    String[] splited = fileName.split("\\.");
    Set<String> set = new HashSet<>();
    for (String str : splited) {
        if (set.add(str) == false) {
           return false;
        }
    }
    return true;
}

Regex for allowing the file extension only once

1 Answers1