Java String.matches（“ regex”）

Question

I don't know regex very well. I am trying to find strings that start with digits 2,3,5 or 7, are 1,3,7 or 9 throughout the middle, and end with 3 or 7.

My attempt was [2357][1379]*[37]. It does not work. I'd appreciate a correction. Remember that this is meant for the Java String.matches() function. Thanks in advance

   for (int s = 0; s < primes.size(); ++s) {
        String p = primes.get(s);

        if (!p.matches([REGEX GOES HERE])) {
            System.out.println(p);
            primes.remove(s);
        }
    }

Answer 1

The standard method of iterating over a collection you remove from in the loop is to iterate downwards that way removals don't affect the index of subsequent elements:

for (int s = primes.size() - 1; s >= 0; s--) {
    String p = primes.get(s);
    if (!p.matches("[2357][1379]*[37]")) {
        System.out.println(p);
        primes.remove(s);
    }
}

No need now to worry about implications of removing elements.

Answer 2

Hi I am not sure I am mistaken .. but i dont see anything wrong in your initial pattern for example

String aa = "[2357][1379]*[37]";
String bb = "2977313";

boolean matches = Pattern.matches(aa, bb); 
System.out.println("1) "+matches);

I started with no 2 and then ended with no 3 and added 1379 in between and it works as expected. Please correct me if i am wrong

Answer 3

The following code "works" just fine:

    List<String> primes = new ArrayList<String>();
    primes.add("1");
    primes.add("2");
    primes.add("7");
    primes.add("23");
    primes.add("213");
    primes.add("243");
    primes.add("2113");
    primes.add("2193");
    for (int s = 0; s < primes.size(); ++s) {
        String p = primes.get(s);

        if (!p.matches("[2357][1379]*[37]")) {
            System.out.println(p);
            primes.remove(s);
        }
    }

It outputs:

1
7
243

You may have expected it to output:

1
2
7
243

However, the primes.remove(s) is messing up your loop. That can't really be the intent of your design. (But who knows?!) The following is one of many solutions to avoid messing up your loop:

    for (String prime : new ArrayList<String>(primes)) {
        if (!prime.matches("[2357][1379]*[37]")) {
            System.out.println(prime);
            primes.remove(prime);
        }
    }

Answer 4

Your regex works well. However, remove shifts any subsequent elements to the left, so the String ordinarily at position s + 1 is moved to s , so the next element to check is at position s instead of s + 1 . Fix:

for (int s = 0; s < primes.size();) {
    String p = primes.get(s);

    if (!p.matches("[2357][1379]*[37]")) {
        System.out.println(p);
        primes.remove(s);
    } else 
        ++s;
}