Java regex "[.]" vs "."

Question

I'm trying to use some regex in Java and I came across this when debugging my code.

What's the difference between [.] and .?

I was surprised that .at would match "cat" but [.]at wouldn't.

Answer 1

[.] matches a dot (.) literally, while . matches any character except newline (\n) (unless you use DOTALL mode).

You can also use \. ("\\." if you use java string literal) to literally match dot.

Answer 2

The [ and ] are metacharacters that let you define a character class. Anything enclosed in square brackets is interpreted literally. You can include multiple characters as well:

[.=*&^$] // Matches any single character from the list '.','=','*','&','^','$'

There are two specific things you need to know about the [...] syntax:

• The ^ symbol at the beginning of the group has a special meaning: it inverts what's matched by the group. For example, [^.] matches any character except a dot .
• Dash - in between two characters means any code point between the two. For example, [A-Z] matches any single uppercase letter. You can use dash multiple times - for example, [A-Za-z0-9] means "any single upper- or lower-case letter or a digit".

The two constructs above (^ and -) are common to nearly all regex engines; some engines (such as Java's) define additional syntax specific only to these engines.