How to Correctly Match Multiline Text with Java Regular Expressions?

Matching Multiline Text with Regular Expressions

Matching multiline text often requires handling line breaks and anchoring. When using Java's Pattern class, there are two modifiers that can assist with this: Pattern.MULTILINE and String.matches(). However, there can be confusion between their usage.

Pattern.MULTILINE vs. (?m)

Pattern.MULTILINE and (?m) both serve the same purpose: allowing ^ and $ to match at the start and end of each line, rather than just at the start and end of the entire string.

String.matches() vs. Pattern.matcher()

String.matches() expects the regular expression to match the entire string. This means its usage can be limited when dealing with multiline text.

Resolving the Example

In the provided example, (?m) is used with String.matches(), which is where the issue lies. Since the regular expression only captures a portion of the multiline text, String.matches() fails to match the entire string and returns false.

Proper Usage for Multiline Matching

To match multiline text correctly, you should use Pattern.compile() with the Pattern.DOTALL modifier, which allows the dot (.) to match newline characters. Here's an updated version of the code:

String test = "User Comments: This is \t a\ta \n test \n\n message \n";
String pattern = "^\s*User Comments:\s*(.*)";
Pattern regex = Pattern.compile(pattern, Pattern.DOTALL);
Matcher regexMatcher = regex.matcher(test);
if (regexMatcher.find()) {
    String result = regexMatcher.group(1);
}

This code will successfully capture the text after "User Comments:" on multiple lines.

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