How Can I Correctly Match Multiline Text Using Java Regular Expressions?

Match Multiline Text Using Regular Expression

When attempting to match multiline text with Java, you may encounter challenges using the Pattern.MULTILINE modifier and the (?m) expression. Here's an example that explains the difference and provides a solution:

We have the following multiline text:

User Comments: This is \t a\ta \n test \n\n message \n

Pattern with Pattern.MULTILINE Modifier:

String pattern1 = "User Comments: (\W)*(\S)*";
Pattern p = Pattern.compile(pattern1, Pattern.MULTILINE);
System.out.println(p.matcher(test).find()); // true

This pattern successfully matches the text because the Pattern.MULTILINE modifier allows the ^ and $ anchors to match at the start and end of each line.

Pattern with (?m) Expression:

String pattern2 = "(?m)User Comments: (\W)*(\S)*";
System.out.println(test.matches(pattern2)); // false

This pattern fails to match because the (?m) expression is incorrect. It should be (?s) to enable the DOTALL mode, which allows the dot (.) to match newline characters.

Furthermore, the matches() method is used to check if the entire string matches the pattern. In this case, the pattern only matches part of the string, so matches() returns false.

Solution:

To correctly match the multiline text using a regular expression, you can use the following pattern with the Pattern.DOTALL modifier:

Pattern regex = Pattern.compile("^\s*User Comments:\s+(.*)", Pattern.DOTALL);
Matcher regexMatcher = regex.matcher(subjectString);
if (regexMatcher.find()) {
    ResultString = regexMatcher.group(1);
}

This pattern will capture the text after "User Comments:" and store it in ResultString.

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