Improve the documentation on escaping in UTF-8 matchers (#4157)

Signed-off-by: George Robinson <[email protected]>

Author: grobinson-grafana

docs/configuration.md

@@ -469,7 +469,7 @@ Alertmanager runs in a special mode called fallback mode as its default mode. As
 In fallback mode, configurations are first parsed as UTF-8 matchers, and if incompatible with the UTF-8 parser, are then parsed as classic matchers. If your Alertmanager configuration contains matchers that are incompatible with the UTF-8 parser, Alertmanager will parse them as classic matchers and log a warning. This warning also includes a suggestion on how to change the matchers from classic matchers to UTF-8 matchers. For example:
 
 ```
-ts=2024-02-11T10:00:00Z caller=parse.go:176 level=warn msg="Alertmanager is moving to a new parser for labels and matchers, and this input is incompatible. Alertmanager has instead parsed the input using the classic matchers parser as a fallback. To make this input compatible with the UTF-8 matchers parser please make sure all regular expressions and values are double-quoted. If you are still seeing this message please open an issue." input="foo=" origin=config err="end of input: expected label value" suggestion="foo=\"\""
+ts=2024-02-11T10:00:00Z caller=parse.go:176 level=warn msg="Alertmanager is moving to a new parser for labels and matchers, and this input is incompatible. Alertmanager has instead parsed the input using the classic matchers parser as a fallback. To make this input compatible with the UTF-8 matchers parser please make sure all regular expressions and values are double-quoted and backslashes are escaped. If you are still seeing this message please open an issue." input="foo=" origin=config err="end of input: expected label value" suggestion="foo=\"\""
 ```
 
 Here the matcher `foo=` can be made into a valid UTF-8 matcher by double quoting the right hand side of the expression to give `foo=""`. These two matchers are equivalent, however with UTF-8 matchers the right hand side of the matcher is a required field.
@@ -517,7 +517,7 @@ Just like Alertmanager server, `amtool` will log a warning if the configuration
 ```
 amtool check-config config.yml
 Checking 'config.yml'
-level=warn msg="Alertmanager is moving to a new parser for labels and matchers, and this input is incompatible. Alertmanager has instead parsed the input using the classic matchers parser as a fallback. To make this input compatible with the UTF-8 matchers parser please make sure all regular expressions and values are double-quoted. If you are still seeing this message please open an issue." input="foo=" origin=config err="end of input: expected label value" suggestion="foo=\"\""
+level=warn msg="Alertmanager is moving to a new parser for labels and matchers, and this input is incompatible. Alertmanager has instead parsed the input using the classic matchers parser as a fallback. To make this input compatible with the UTF-8 matchers parser please make sure all regular expressions and values are double-quoted and backslashes are escaped. If you are still seeing this message please open an issue." input="foo=" origin=config err="end of input: expected label value" suggestion="foo=\"\""
 level=warn msg="Matchers input has disagreement" input="qux=\"\\xf0\\x9f\\x99\\x82\"\n" origin=config
   SUCCESS
 Found:
@@ -575,9 +575,9 @@ A UTF-8 matcher consists of three tokens:
 - One of `=`, `!=`, `=~`, or `!~`. `=` means equals, `!=` means not equal, `=~` means matches the regular expression and `!~` means doesn't match the regular expression.
 - An unquoted literal or a double-quoted string for the regular expression or label value.
 
-Unquoted literals can contain all UTF-8 characters other than the reserved characters. These are whitespace, and all characters in ``` { } ! = ~ , \ " ' ` ```. For example, `foo`, `[a-zA-Z]+`, and `Προμηθεύς` (Prometheus in Greek) are all examples of valid unquoted literals. However, `foo!` is not a valid literal as `!` is a reserved character.
+Unquoted literals can contain all UTF-8 characters other than the reserved characters. The reserved characters include whitespace and all characters in ``` { } ! = ~ , \ " ' ` ```. For example, `foo`, `[a-zA-Z]+`, and `Προμηθεύς` (Prometheus in Greek) are all examples of valid unquoted literals. However, `foo!` is not a valid literal as `!` is a reserved character.
 
-Double-quoted strings can contain all UTF-8 characters. Unlike unquoted literals, there are no reserved characters. You can even use UTF-8 code points. For example, `"foo!"`, `"bar,baz"`, `"\"baz qux\""` and `"\xf0\x9f\x99\x82"` are valid double-quoted strings.
+Double-quoted strings can contain all UTF-8 characters. Unlike unquoted literals, there are no reserved characters. However, literal double quotes and backslashes must be escaped with a single backslash. For example, to match the regular expression `\d+` the backslash must be escaped `"\\d+"`. This is because double-quoted strings follow the same rules as Go's [string literals](https://go.dev/ref/spec#String_literals). Double-quoted strings also support UTF-8 code points. For example, `"foo!"`, `"bar,baz"`, `"\"baz qux\""` and `"\xf0\x9f\x99\x82"`.
 
 #### Classic matchers