Regex for italic markdown

Question

I'm trying for hours with regex: I need a regex to select all that is inside underlines. Example:

\_italic\_

But with the only condition that I need it to ignore \\_ (backslash followed by underscore).

So, this would be a match (all the text which is inside the \_):

\_italic some text 123 \\_*%&$ _

SO far I have this regex:

(\_.*?\_)(?!\\\_) 

But is not ignoring the \\_

Which regex would work?

Answer 1

You can use

(?s)(?<!\\)(?:\\{2})*_((?:[^\\_]|\\.)+)_

See the regex demo. Details:

• (?s) - an inline embedded flag option equal to Pattern.DOTALL
• (?<!\\)(?:\\{2})* - a position that is not immediately preceded with a backslash and then zero or more sequences of double backslashes
• _ - an underscore
• ((?:[^\\_]|\\.)+) - Capturing group 1: one or more occurrences of any char other than a \ and _, or any escaped char (a combination of a \ and any one char)
• _ - an underscore

See the Java demo:

List<String> strs = Arrays.asList("xxx _italic some text 123 \\_*%&$ _ xxx",
                                          "\\_test_test_");
String regex = "(?s)(?<!\\\\)(?:\\\\{2})*_((?:[^\\\\_]|\\\\.)+)_";
Pattern p = Pattern.compile(regex);
for (String str : strs) {
    Matcher m = p.matcher(str);
    List<String> result = new ArrayList<>();
    while(m.find()) {
        result.add(m.group(1));
    }
    System.out.println(str + " => " + String.join(", ", result));
}

Output:

xxx _italic some text 123 \_*%&$ _ xxx => italic some text 123 \_*%&$ 
\_test_test_ => test