java - 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.

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.