Question

In the following let’s say zip codes I am trying to exclude the 33333- from the result.
I do:

String zip = "11111 22222 33333- 44444-4444";
String regex = "\\d{5}(?(?=-)-\\d{4})";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(zip);
while (matcher.find()) { 
   System.out.println(" Found: " + matcher.group());     
}

Expect to get:

Found:  11111  
Found:  22222  
Found:  44444-4444

I am trying to enforce format of:
5 digits optionally followed by a – and 4 digits. 5 digits with just a – (hyphen) is not wanted

I get exception:

Exception in thread "main" java.util.regex.PatternSyntaxException: Unknown inline modifier near index 7
\d{5}(?(?=-)(-\d{4}))
       ^
    at java.util.regex.Pattern.error(Unknown Source)
    at java.util.regex.Pattern.group0(Unknown Source)
    at java.util.regex.Pattern.sequence(Unknown Source)
    at java.util.regex.Pattern.expr(Unknown Source)
    at java.util.regex.Pattern.compile(Unknown Source)
    at java.util.regex.Pattern.<init>(Unknown Source)
    at java.util.regex.Pattern.compile(Unknown Source)

Am I not using the conditional lookahead correctly?

Answer 1

To capture all numbers except 33333 use this code:

String zip = "11111 22222 33333- 44444-4444";
String regex = "\\d{5}(?=(-\\d{4}|\\s|$))(-\\d{4})?";
Matcher m = Pattern.compile(regex).matcher(zip);
while(m.find())
    System.out.printf("Macthed: [%s]%n", m.group(1));

OUTPUT:

Macthed: [11111]
Macthed: [22222]
Macthed: [44444-4444]

Explanation: This RegEx is using lookahead that itself is like a condition, which means match 5 digit number which must be followed by – and 4 digits OR a space OR end of string and then it is optionally matching a text – and 4 digits.

The reason why your original RegEx is throwing exception because there is a syntax error in ?:(?=-) part of your RegEx.