How Do `Pattern.MULTILINE` and `(?m)` Differ When Matching Multiline Text in Java Regular Expressions?

Match Multiline Text Using Regular Expression

When attempting to match multiline text in Java, you may encounter differences in behavior between using the Pattern class with the Pattern.MULTILINE modifier and using the (?m) modifier.

To understand the discrepancy, it's crucial to grasp the purpose of these modifiers.

Pattern.MULTILINE and (?m)

Both Pattern.MULTILINE and (?m) are used to extend the behavior of regular expression anchors (^ and $) to match not only the start and end of the entire string but also the start and end of each line within the string.

Pattern.DOTALL and (?s)

However, the key difference between these modifiers lies in the handling of newline (carriage return) characters. Pattern.MULTILINE does not extend the matching ability of the dot (.) wildcard character to include newline characters. To include them, you must use Pattern.DOTALL or (?s).

Matching the Example String

In your example, the string contains multiple lines, and you want to find text that starts with "User Comments:". Using Pattern.MULTILINE alone will allow the anchors to match the start of each line, but the dot character will not match newlines.

To correctly match multiline text and capture the characters after "User Comments:", you should use Pattern.DOTALL or (?s) in conjunction with Pattern.MULTILINE or (?m):

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

This regex will match the start of each line and will further capture all non-whitespace characters after "User Comments:".

The above is the detailed content of How Do `Pattern.MULTILINE` and `(?m)` Differ When Matching Multiline Text in Java Regular Expressions?. For more information, please follow other related articles on the PHP Chinese website!