What do ^ and $ mean in a regular expression?

Question

What is the difference between "\\w+@\\w+[.]\\w+" and "^\\w+@\\w+[.]\\w+$"? I have tried to google for it but no luck.

Travesty

did you try your strings and regex with regex101.com? – Melodious

Answer 1

^ means "Match the start of the string" (more exactly, the position before the first character in the string, so it does not match an actual character).

$ means "Match the end of the string" (the position after the last character in the string).

Both are called anchors and ensure that the entire string is matched instead of just a substring.

So in your example, the first regex will report a match on [email protected], but the matched text will be [email protected], probably not what you expected. The second regex will simply fail.

Be careful, as some regex implementations implicitly anchor the regex at the start/end of the string (for example Java's .matches(), if you're using that).

If the multiline option is set (using the (?m) flag, for example, or by doing Pattern.compile("^\\w+@\\w+[.]\\w+$", Pattern.MULTILINE)), then ^ and $ also match at the start and end of a line.

Gradey

So if I Understand correctly then the string "[email protected]" is perfectly matched with "\\w+@\\w+[.]\\w+" but not "^\\w+@\\w+[.]\\w+$"? but I have tested in java and the string failed with both cases. I still do not see the clear difference. Can you show the points based on my case? – Travesty

Your regex only allows for one dot after the @ sign. Try ^[\\w.]+@[\\w.]+\\.\\w+$. It's still not perfect (no regex will ever be for matching an e-mail address), but it's a bit more forgiving. – Gradey

yes, according to your answer, the string "[email protected]" would be matched with "\\w+@\\w+[.]\\w+", but not with "^\\w+@\\w+[.]\\w+$". This code: String s = "[email protected]"; System.out.println(s.matches("\\w+@\\w+[.]\\w+")); => false – Travesty

Did you read my answer thoroughly? Java's .matches() adds anchors to your regex implicitly! – Gradey

Answer 2

Try the Javadoc:

http://download.oracle.com/javase/6/docs/api/java/util/regex/Pattern.html

^ and $ match the beginnings/endings of a line (without consuming them)

Osyth