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e443. Matching Line Boundaries in a Regular Expression

By default, the beginning-of-line matcher (^) and end-of-line matcher ($) do not match at line boundaries. They match the beginning and end of the entire input sequence. For example, the pattern ^a matches abc but does not match def\nabc. To enable ^ and $ to match line boundaries, the pattern should be compiled with the multiline flag enabled.

It is also possible to enable multiline mode within a pattern using the inline modifier (?m). For example, multiline mode is enabled in the pattern (?m)^a. Multiline mode can be disabled using (?-m).

    CharSequence inputStr = "abc\ndef";
    String patternStr = "abc$";
    
    // Compile with multiline enabled
    Pattern pattern = Pattern.compile(patternStr, Pattern.MULTILINE);
    Matcher matcher = pattern.matcher(inputStr);
    boolean matchFound = matcher.find();    // true
    
    // Use an inline modifier to enable multiline mode
    matchFound = pattern.matches(".*abc$.*", "abc\r\ndef");     // false
    matchFound = pattern.matches("(?m).*abc$.*", "abc\r\ndef"); // true