"Unknown escape sequence" error in Go

The \ backslash isn't being interpreted by the regex parser, it's being interpreted in the string literal. You should escape the backslash again:

regexp.Compile("[0-9]+\\.[0-9]+\\.[0-9]+\\.[0-9]+")

A string quoted with " double-quote characters is known as an "interpreted string literal" in Go. Interpreted string literals are like string literals in most languages: \ backslash characters aren't included literally, they're used to give special meaning to the next character. The source must include \\ two backslashes in a row to obtain an a single backslash character in the parsed value.

Go has another alternative which can be useful when writing string literals for regular expressions: a "raw string literal" is quoted by ` backtick characters. There are no special characters in a raw string literal, so as long as your pattern doesn't include a backtick you can use this syntax without escaping anything:

regexp.Compile(`[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+`)

These are described in the "String literals" section of the Go spec.

IPv4 address (accurate capture)

Matches 0.0.0.0 through 255.255.255.255

Use this regex to match IP numbers with accurracy.

Each of the 4 numbers is stored into a capturing group, so you can access them for further processing.

"(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\\.(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\\.(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\\.(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])"