How does Go\'s ReplaceAllString function handle the replacement of \'$1W\' when the capture group \'1W\' is not defined in the regular expression?

Understanding the Output of Go's ReplaceAllString Function

The ReplaceAllString function in Go provides a way to replace all matches of a regular expression within a string. While the first output of the example code provided is straightforward to comprehend, the subsequent outputs may seem puzzling.

Output 2 and 4: Understanding $1 Backreferences

In the second and fourth outputs, $1 refers to the capture group defined by the first set of parentheses in the regular expression: a(x)b. This capture group matches and captures the string made up of any number of 'x' characters.

• In Output 2 (fmt.Println(re.ReplaceAllString("-ab-axxb-", "$1"))), $1 is replaced with the captured string of 'x' characters (--xx-).
• In Output 4 (fmt.Println(re.ReplaceAllString("-ab-axxb-", "${1}W"))), $1 is replaced with the same captured string, but enclosed in curly braces (-W-xxW-).

Output 3: Understanding $1W

The third output (fmt.Println(re.ReplaceAllString("-ab-axxb-", "$1W"))) is the most intriguing. The documentation on ReplaceAllString states that "$ signs" in the replacement pattern are interpreted as in the Expand function:

Inside repl, $ signs are interpreted as in Expand

Expand specifies that:

In the template, a variable is denoted by a substring of the form $name or ${name}, where name is a non-empty sequence of letters, digits, and underscores.
A reference to an out of range or unmatched index or a name that is not present in the regular expression is replaced with an empty slice.
In the $name form, name is taken to be as long as possible: $1x is equivalent to ${1x}, not ${1}x, and, $10 is equivalent to ${10}, not ${1}0.

In our case, $1W is equivalent to ${1W}, as it is taken to be as long as possible. However, the capture group 1W is not defined in the regular expression. This means that it was not populated during the matching operation, and thus, it is considered an "out of range" or "unmatched" index. Therefore, $1W is replaced with an empty string ("") during the replacement phase.

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