java.util.regex.Pattern flags (for example CASE_INSENSITIVE) are ignored when RegexFilter is created

Describe the bug

When creating a RegexFilter, flags from passed java.util.regex.Pattern are ignored.

To Reproduce

Create a query with a RegexFilter created from a java.regex.Pattern with CASE_INSENSITIVE flag set, for example:

    Query<Unit> query = datastore.find(Unit.class);
    String patternString = String.format(".*%s.*", searchString);
    Pattern pattern = Pattern.compile(patternString, Pattern.CASE_INSENSITIVE);
    query.filter(regex("name", pattern));

Perform the query with a string matching case-insensitively but not case-sensitively.

Expected behavior

Entity (matching case-insensitively) is returned.

Please complete the following information:

• Server Version: 4.4.28
• Driver Version: 5.6.1
• Morphia Version: 2.5.1

Additional context

In Morphia 1.3.2 the corresponding filter creation

    Pattern pattern = Pattern.compile(patternString, Pattern.CASE_INSENSITIVE);
    query.filter("name", pattern);

added case-insensitive option to regex in resulting query.

{ query: {"name": {"$regex": ".*any.*", "$options": "i"}} }

Workaround

Add case-insensitivity to the pattern string by prepending (?i).

Start with RegexFilter creation and follow how the passed java.util.regex.Pattern is converted into the MongoDB regex query. Reproduce the example using Pattern.CASE_INSENSITIVE and verify that a case-insensitive match is returned and the resulting query includes the equivalent option, without requiring the (?i) workaround.