How to use regular expression in some part of string in java?

There are a variety of ways you can accomplish this task

Regex

The pattern

"emp-id\\d+"

should achieve what you're wanting for both cases. The pattern matches "emp-id" plus 1 or more digits (\\d+).

public static void main(String[] args) throws Exception {
    String case1 = "project/emp-id1545/ID-JHKDKHENNDHJSJ";
    String case2 = "project/**dep**/emp-id8545/ID-GHFRDEEDE";

    Matcher matcher = Pattern.compile("emp-id\\d+").matcher(case1);
    // Changed from while to if cause we're only going to get the first match
    if (matcher.find()) {
        System.out.println(matcher.group());
    }

    matcher = Pattern.compile("emp-id\\d+").matcher(case2);
    // Changed from while to if cause we're only going to get the first match
    if (matcher.find()) {
        System.out.println(matcher.group());
    }
}

Results:

emp-id1545
emp-id8545

Java 8

Given that your data indicates that the character "/" is a delimiter. You can also use String.split() and Stream.filter() (Java 8) to find your String.

public static void main(String[] args) throws Exception {
    String case1 = "project/emp-id1545/ID-JHKDKHENNDHJSJ";
    String case2 = "project/**dep**/emp-id8545/ID-GHFRDEEDE";

    System.out.println(Arrays.stream(case1.split("/")).filter(s -> s.startsWith("emp-id")).findFirst().get());
    System.out.println(Arrays.stream(case2.split("/")).filter(s -> s.startsWith("emp-id")).findFirst().get());
}

Results:

emp-id1545
emp-id8545

Non Regex or Java 8

Still using "/" delimiter and "emp-id" you can use String.indexOf() and String.substring() to extract the String you're looking for.

public static void main(String[] args) throws Exception {
    String case1 = "project/emp-id1545/ID-JHKDKHENNDHJSJ";
    String case2 = "project/**dep**/emp-id8545/ID-GHFRDEEDE";

    int index = case1.indexOf("emp-id");
    System.out.println(case1.substring(index, case1.indexOf("/", index)));

    index = case2.indexOf("emp-id");
    System.out.println(case2.substring(index, case2.indexOf("/", index)));
}

Results:

emp-id1545
emp-id8545