Question

The regular expression seems to be ok, since the first line line correctly replace the substring with “helloworld”, but the same expression won’t match in the latter since i cannot see “whynothelloworld?” on console

System.out.println(current_tag.replaceAll("^[01][r]\\s", "helloworld"));

if (Pattern.matches("^[01][r]\\s", current_tag)) { System.out.println("whynothelloworld?");}

Answer 1

Pattern.matches() expects the entire string to match, not just a substring.

Use the .find() method of the regex matcher object instead:

Pattern regex = Pattern.compile("^[01]r\\s");
Matcher regexMatcher = regex.matcher(current_tag);
foundMatch = regexMatcher.find();