Java library for generating regular expression from input string

Question

There are many websites which take a string as user input and allow you to create a regular expression (regex) from pieces of the string.

But I could not find any java library which does the same. Is there any Java library available which generates a regular expression that exactly matches a string?

String inputString = "ABC345";
String regularExpression = Something.generateRegEx(inputString);

or something like that.

Note: I have a condition wherein I want to take some string from user, generate regular expression and then match for that pattern on some data-sets to extract similar patterns. I have created a small utility, but it is not that reliable yet. Moreover, I am looking for some well-tested library.

EDIT :

Please visit txt2re.com. I want a java library which performs the same function.

Do you just want to escape the input? See this question: stackoverflow.com/questions/60160/… and the answer regarding quote method. — Dave L.
– Dave L., Commented Jul 30, 2012 at 14:21
I used to write a library that do this. It always return .* — Kien Truong
– Kien Truong, Commented Jul 30, 2012 at 14:24
@ all - sorry for the inadequate description. I have just updated the question. — Saurabh
– Saurabh, Commented Jul 30, 2012 at 14:31

Louis Wasserman · Accepted Answer · 2012-07-30 14:24:12Z

3

Pattern.quote(String) returns a (string) regex that matches the specified string exactly.

answered Jul 30, 2012 at 14:24

Louis Wasserman

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5 Comments

Saurabh Over a year ago

I have just used Pattern.quote(String) and the output was \QABC123\E

Louis Wasserman Over a year ago

...Yes? What's the problem with that?

Chris Dargis Over a year ago

@Saurabh: Is this a problem? What was your input?

Saurabh Over a year ago

@DougRamsey : sorry for inadequate information. I have just updated my question.

Louis Wasserman Over a year ago

I can't figure out what that site is doing. Can you clarify the question yourself, rather than pointing to another source?

Erhan Bagdemir · Accepted Answer · 2012-07-30 15:06:21Z

2

I think, the txt2re.com has a database from known regular expressions, since the tool extends its answers with semantics like "date" or "email" for date and email formats. Otherwise, it gives an expression, which validates only a string but not a "regular language". Regular languages are expressed by regular expressions and they can be calculated by finite-state machines, but they are sets of limited words (all finite languages are regular). For example a simple language like:

L = { (a^n)(b^n) | n >= 0 } is not regular. (proof with pumping lemma)

L = {ab, aabb, aaabbb,...} (not- regular)

if you consider, that the input is a set of infinite words (inclusive natural languages), however, the regular expressions can not describe all of them. In order to generate regular expressions for a language, you had to first describe it with a (TYPE-3) grammar.

if your language has only a word like this:

L = { [email protected] }

then you can write a basic compiler iterating over the chars while checking their types, pseudo:

s = size(input) 
result = ""
for (i = 0; i < s; i++) {
   if input[i] is numeric
      result += "d"
   else if input[i] is word
      result += "w" 
   ...
}
return result

answered Jul 30, 2012 at 15:06

Erhan Bagdemir

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1 Comment

Saurabh Over a year ago

Thanks for your detailed answer. So, there's no such library already available. And to make such library, one need to have database of known regular expressions included in that library. rite? Thanks for your pseudo code, in fact my current running code (work-around) uses same logic for generating regular expression.

Aayush · Accepted Answer · 2019-05-10 03:14:25Z

0

A genetic algorithm based java library like regex++ url: https://github.com/MaLeLabTs/RegexGenerator can be used for the same purpose.

answered May 10, 2019 at 3:14

Aayush

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Comments

kgautron · Accepted Answer · 2012-07-30 14:41:01Z

-1

If what you want is to find a regex matching a given String, this does not make sense because there exists an infinite number of it.

On a contrary if you want to build a Pattern object from a regex that is input from the user, use the standard java API (java.util.regex.*) this way :

Pattern p = Pattern.compile(inputString);

edited Jul 30, 2012 at 14:41

answered Jul 30, 2012 at 14:20

kgautron

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7 Comments

Erhan Bagdemir Over a year ago

he wants to get regular expressions from a string given.

kgautron Over a year ago

@ErhanBagdemir yes, so what is the problem?

Erhan Bagdemir Over a year ago

Pattern.compile takes the regular expression as parameter, but it doesn't give the expression self for a given string.

kgautron Over a year ago

How could that be possible? For any String there is an infinite number of regex matching it.

Erhan Bagdemir Over a year ago

an answer like your latest comment could get +1. but the current answer -1.

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