Greedy search for string patterns using regular expressions in Java

Question

I have a use case where I want to search for a operator amongst <,>,<=,>=, and = in a given string and split the expression into 2 parts i.e. the right expression and the left expression and evaluate them individually before evaluating the final conditional operator.

This can be understood from the example below:

Pattern pattern1 = Pattern.compile("(.*?)(<|>|<=|=|>=)(.*)");
Matcher matcher2 = pattern1.matcher("4>=5");
while (matcher2.find()) {
        System.out.println(matcher2.group(1) + ";" + matcher2.group(2)+ ";" + matcher2.group(3));
}

Output:

4;>;=5

The expected output was 4;>=;5 but the >= operator got split because of the presence of the operator > independently.

I want to evaluate the clause (<|>|<=|=|>=) in a greedy fashion so that >= gets treated as a single entity and gets listed down if they occur together.

Just put them in the right order ... (<=|>=|=|>|<). That should do what you want. — Robert Hanson
– Robert Hanson, Commented Oct 26, 2012 at 20:27
@RobertHanson - Thats a good one. It worked but what if I am getting a list of these operators from a list. In that case, I may not be sure of the order. How do I make it independent of the order? — Abhishek Jain
– Abhishek Jain, Commented Oct 26, 2012 at 20:28
@AbhishekJain - You could sort them by length first. E.g. Collections.sort(listOfOperators, lengthComparator), where lengthComparator is an instance of Comparator that has been implemented to sort by length. — Robert Hanson
– Robert Hanson, Commented Oct 27, 2012 at 19:59
@RobertHanson - Thanks for suggesting that but I have already implemented the same using the same technique. Anyways, thanks for posting this. :) — Abhishek Jain
– Abhishek Jain, Commented Oct 27, 2012 at 20:20

Rohit Jain · Accepted Answer · 2012-10-26 20:33:26Z

2

you can try simplifying to

 pattern1 = Pattern.compile("(.*?)(>=?|<=?|=)(.*)");

edited Oct 26, 2012 at 20:33

Rohit Jain

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answered Oct 26, 2012 at 20:30

shyam

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4 Comments

Rohit Jain Over a year ago

+1 Nice simplification rather. But this doesn't work for 4=5.

Abhishek Jain Over a year ago

@shyam - what if I am getting the operator list from an unsorted data structure. I may not be able to express the same without knowing all the operators.

shyam Over a year ago

@AbhishekJain Then you should not use regular expressions. What if one of your operators had a character that has special meaning as a regex?

shyam Over a year ago

@RohitJain Thanks :) missed the = in all those pipes

Javier Diaz · Accepted Answer · 2012-10-26 20:32:48Z

1

String testt = "4>=5";
System.out.println(testt.replaceAll("(.*?)(>=?|<=?|=)(.*)", "$1;$2;$3"));

Easy to understand and you will replace all at once. You had a mistake that would stop getting <= if it finds a < before it, so just place those 2 <= and >= to the first places.

answered Oct 26, 2012 at 20:32

Javier Diaz

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8 Comments

Abhishek Jain Over a year ago

same comment as I made to other answers as well : what if I am getting the operator list from an unsorted data structure. I may not be able to express the same without knowing all the operators.

Javier Diaz Over a year ago

So why not make a (operator1|operator2) string (runtime) and then attach it to the regex? "(.*?)"+operators+"(.*)"

Abhishek Jain Over a year ago

exactly what I am trying to do but 1) I may not be sure of the order in which the operators come in from the unsorted data structure so, the order may well be <,>,<=,>= and thus the same problem. 2) I want to make it a dynamic thing where the operators may not be inter-related.

Abhishek Jain Over a year ago

Yes, even I was thinking about that option. Let me give it a try. :)

Abhishek Jain Over a year ago

But this would be an ugly implementation. Can we not do something smarter?

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