replaceFirst in a regex with $1 - JAVA

Question

I need to extract the following pattern from a string and return one of the possible matches according to the input.

I used regular expressions, through different possibilities and I have not managed to get the expected result:

input a): 71346 G249 USD 70045620 27/08/2020 001 / 004

input b): 71346 G249 USD 70045620/2020 27/08/2020 001 / 004

Try one

String result = data.replaceFirst ( "(.*?([0-9]{6,}\\/[0-9]{4}).*)|(.*?([0-9]{6,}).*)", "$1" );

Try two

String result = data.replaceFirst ( ".*?([0-9]{6,}\\/[0-9]{4})|([0-9]{6,}).*", "$1" );

Try three

String result = data.replaceFirst ( ".*?([0-9]{6,})([0-9]{6,}\\/[0-9]{4}).*", "$1" );

Expected result according to input:

input a): 70045620

input b): 70045620/2020

Answer 1

Using an alternation with capturing groups in that way will give you different groupnumbers depending on the data. If you want a single group in the replacement, you can make the second part optional.

String[] strings = { 
    "71346 G249 USD 70045620 27/08/2020 001 / 004",
    "71346 G249 USD 70045620/2020 27/08/2020 001 / 004"
};
String regex = "^.*?\\b([0-9]{6,}(?:/[0-9]{4})?)\\b.*$";

for (String s : strings) {
    System.out.println(s.replaceFirst(regex, "$1"));
}

See a Java demo

Output

70045620
70045620/2020

---

You could also find the match instead of using replaceFirst.

\b[0-9]{6,}(?:/[0-9]{4})?

For example

String[] strings = { 
    "71346 G249 USD 70045620 27/08/2020 001 / 004",
    "71346 G249 USD 70045620/2020 27/08/2020 001 / 004"
};
String regex = "\\b[0-9]{6,}(?:/[0-9]{4})?";    
Pattern pattern = Pattern.compile(regex);    

for (String s : strings) {
    Matcher matcher = pattern.matcher(s);
    if (matcher.find()) {
        System.out.println(matcher.group(0));
    }
}

See another Java demo

Output

70045620
70045620/2020