Generating a regex from a string

Question

For example I have a string "anet.leverling@gmail.com" so regex pattern of this string would be "[a-t]{4}.[a-z]{9}@[a-z]{5}.[a-z]{3}" I want to know how to write a java code to generate that regex according to the given string.

Answer 1

In the general sense, you are asking for the impossible.

1. There is literally an infinite number of distinct regexes that would match that (single) example string. The specific regex you have provided is no more correct or incorrect than many others. (Indeed, all of them ... if you don't provide criteria for ranking the regexes for correctness!)

2. If you only provide a single example string, there is nothing to say whether the generated regex will work with "similar" strings, or exclude strings that are not "similar". And you haven't said what "similar" would mean.

Actually ... there is a fatuous solution to your requirements.

   public String generatePattern(String input) {
       return ".*";
   }

The ".*" regex will match any string, and therefore satisfies your requirement for a regex that will match your input string.

And of you want a regex that is a perfect match for only the input and nothing else, that is also fatuously simple.

  public String generatePattern(String input) {
       return "\Q" + input + "\E";
  }

or just call Pattern.quote(input) to get a Pattern that encodes an equivalent regex.

---

To help you understand why your vague "according to the string" requirement is problematic, consider the following alternative regexes for that input.

[a-t]{4}.[a-z]{9}@[a-z]{5}.[a-z]{3}               // Your version
[a-t]{4}\.[a-z]{9}@[a-z]{5}\.[a-z]{3}             // We want '.' specifically
                                                  // rather than any single char 
[a-z]{4}.[a-z]{9}@[a-z]{5}.[a-z]{3}               // Why exclude uvwxyz from the
                                                  // the first 'name'
[a-t]{4}.[e-v]{9}@[a-z]{5}.[a-z]{3}               // Why allow 'abcd' or 'wxyz'
                                                  // in the second 'name'
[a-zA-Z]{4}\.[a-zA-Z]{9}@[a-zA-Z]{5}.[a-zA-Z]{3}  // uppercase letters
                                                  // OK in email addresses

Basically, if you want to say that your version is correct and the others are incorrect, then your requirements need to include a whole set of rules that allows the developer to distinguish the correct from the incorrect.

But if you do have those rules, then it is relatively straight forward to determine what pattern an example represents and generate the corresponding regex.