Question

How can i write this as a regular expression?

“blocka#123#456”

i have used # symbol to split the parameters in the data
and the parameters are block name,startX coordinate,start Y corrdinate

this is the data embedded in my QR code.so when i scan the QR i want to check if its the right QR they’re scanning. For that i need a regular expression for the above syntax.

my method body

public void Store_QR(String qr){
   if( qr.matches(regular Expression here)) {
      CurrentLocation = qr;
   }
   else // Break the operation
}

Answer 1

myregexp.com is nice to do some testing.

Official Java Regex Tutorial is quite ok to learn and includes most things one needs to know.

The Pattern documentation also includes fancy predefined character classes that are missing in above tutorial.

You did not specify anything that has to be regular in that example you gave. Regular expressions make only sense if there are rules to validate the input.

If it has to be exactly "blocka#123#456" then "blocka#123#456" or "^blocka#123#456$" will work as regex. Stuff between ^ and $ means that the regex inside must span from begin to end of the input. Sometimes required and usually a good idea to put that around your regex.

If blocka is dynamic replace it with [a-z]+ to match any sequence of lowercase letters a through z with length of at least 1. block[a-z] would match blocka, blockb, etc.
And [a-z]{6} would match any sequence of exactly 6 letters. [a-zA-Z] also includes uppercase letters and \p{L} matches any letter including unicode stuff (e.g. Blüc本).

# matches #. Like any character without special regex meaning ( \ ^ $ . | ? * + ( ) [ ] { } ) characters match themselves. [^#] matches every character but #.

Regarding the numbers: [0-9]+ or \d+ is a generic pattern for several numbers, [0-9]{1,4} would match anything consisting out of 1-4 numbers like 007, 5, 9999. (?:0|[1-9][0-9]{0,3}) for example will only match numbers between 0 and 9999 and does not allow leading zeros. (?:STUFF) is a non-capturing group that does not affect the groups you can extract via Matcher#group(1..?). Useful for logical grouping with |. The meaning of (?:0|[1-9][0-9]{0,3}) is: either a single 0 OR ( 1x 1–9 followed by 0 to 3 x 0–9).

[0-9] is so common that there is a predefinition for it : \d (digit). It’s \\d inside the regex String since you have to escape the \.

So some of your options are

• ".*" which matches absolutely everything
• "^[^#]+(?:#[^#]+)+$" which matches anything separated by # like "hello #world!1# -12.f #本#foo#bar"
• "^blocka(#\\d+)+$" which matches blocka followed by at least one group of numbers separated by # e.g. blocka#1#12#0007#949432149#3
• "^blocka#(?:[0-9]|[1-9][0-9]|[1-3][0-9]{2})#[4-9][0-9]{2}$" which will match only if it finds blocka# followed by numbers 0 – 399, followed by a # and finally numbers 400-999
• "^blocka#123#456$" which matches only exactly that string.

All that are regular expressions that match the example you gave.

But it’s probably as simple as

public void Store_QR(String qr){
   if( qr.matches("^blocka#\\d+#\\d+$")) {
      CurrentLocation = qr;
   }
   else // Break the operation
}

or

private static final Pattern QR_PATTERN = Pattern.compile("^blocka#(\\d+)#(\\d+)$");
public void Store_QR(String qr){
    Matcher matcher = QR_PATTERN.matcher(qr);
    if(matcher.matches()) {
        int number1 = Integer.valueOf(matcher.group(1));
        int number2 = Integer.valueOf(matcher.group(2));
       CurrentLocation = qr;
    }
    else // Break the operation
 }

---

BlockName#start_X#start_Y any block name.. starting with the string”block” and followed by two integers

I guess a good regex for that would be "^block\\w+#\\d+#\\d+$", starting with "block", then any combination of a–z, A–Z, 0–9 and _ (thats the \w) followed by #, numbers, #, numbers.

Would match block_#0#0, blockZ#9#9, block_a_Unicorn666#0000#1234, but not block#1#2 because there is no name at all and would not match blockName#123#abc because letters instead of number. Would also not match Block_a#123#456 because of the uppercase B.

If the name part (\\w+) is too liberal (___, _123 would be a legal names) use e.g. "^block_?[a-zA-Z]+#\\d+#\\d+$", what won’t allow numbers and names may only be separated by a single optional _ and there have to be letters after that. Would allow _a, a, _ABc, but not _, _a_b, _a9. If you want to allow numbers in names [a-zA-Z0-9] would be the character class to use.