Java Regular expression validation

Question

I want to validate a string which allows only alpha numeric values and only one dot character and only underscore character in java .

String fileName = (String) request.getParameter("read");

I need to validate the fileName retrieving from the request and should satisfy the above criteria

I tried in "^[a-zA-Z0-9_'.']*$" , but this allows more than one dot character

I need to validate my string in the given scenarios ,

1 . Filename contains only alpha numeric values . 2 . It allows only one dot character (.) , example : fileRead.pdf , fileWrite.txt etc 3 . it allows only underscore characters . All the other symbols should be declined

Can any one help me on this ?

regex101.com this online service lets you try out regex and explains the single characters and what they do in a regex really nice. It's very good for self study :) — Herr Derb
– Herr Derb, Commented Jun 16, 2017 at 8:41
you should point whether the filename contains file extension. your pattern allows last dot and first dot. — mattn
– mattn, Commented Jun 16, 2017 at 8:54
You do realize that both Windows and Unix/Linux/OSX allow filenames with more than one dot character? Is there a specific purpose for your special restriction? — Klitos Kyriacou
– Klitos Kyriacou, Commented Jun 16, 2017 at 9:19

Valentin Genevrais · Accepted Answer · 2017-06-16 09:09:49Z

20

You should use String.matches() method :

System.out.println("My_File_Name.txt".matches("\\w+\\.\\w+"));

You can also use java.util.regex package.

java.util.regex.Pattern pattern = 
java.util.regex.Pattern.compile("\\w+\\.\\w+");

java.util.regex.Matcher matcher = pattern.matcher("My_File_Name.txt");

System.out.println(matcher.matches());

For more information about REGEX and JAVA, look at this page : https://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html

edited Jun 16, 2017 at 9:09

answered Jun 16, 2017 at 8:49

Valentin Genevrais

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4 Comments

Olivier Grégoire Over a year ago

Don't use matcher.find() but matcher.matches(). The regex part of your answer accepts input like ßαd.txt (or worse: .........a.txt). If you use matches(), that won't be the case anymore.

mattn Over a year ago

you must escape backslash double.

Valentin Genevrais Over a year ago

Thanks you Olivier

Valentin Genevrais Over a year ago

Thanks you mattn :)

Tim Biegeleisen · Accepted Answer · 2017-06-16 08:49:46Z

1

You could use two negative lookaheads here:

^((?!.*\..*\.)(?!.*_.*_)[A-Za-z0-9_.])*$

Each lookahead asserts that either a dot or an underscore does not occur two times, implying that it can occur at most once.

It wasn't completely clear whether you require one dot and/or underscore. I assumed not, but my regex could be easily modified to this requirement.

Demo

answered Jun 16, 2017 at 8:49

Tim Biegeleisen

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Comments

ahmetcetin · Accepted Answer · 2017-06-16 08:44:24Z

0

You can first check the special characters which have the number limits. Here is the code:

int occurance = StringUtils.countOccurrencesOf("123123..32131.3", ".");

or

int count = StringUtils.countMatches("123123..32131.3", ".");

If it does not match your request you can discard it before regex check. If there is no problem you can now put your String to alphanumeric value check.

answered Jun 16, 2017 at 8:44

ahmetcetin

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