Improve the RegEx class documentation

This closes godotengine/godot-docs#2522.

Author: Calinou

modules/regex/doc_classes/RegEx.xml

@@ -10,7 +10,7 @@
 		var regex = RegEx.new()
 		regex.compile("\\w-(\\d+)")
 		[/codeblock]
-		The search pattern must be escaped first for gdscript before it is escaped for the expression. For example, [code]compile("\\d+")[/code] would be read by RegEx as [code]\d+[/code]. Similarly, [code]compile("\"(?:\\\\.|[^\"])*\"")[/code] would be read as [code]"(?:\\.|[^"])*"[/code].
+		The search pattern must be escaped first for GDScript before it is escaped for the expression. For example, [code]compile("\\d+")[/code] would be read by RegEx as [code]\d+[/code]. Similarly, [code]compile("\"(?:\\\\.|[^\"])*\"")[/code] would be read as [code]"(?:\\.|[^"])*"[/code].
 		Using [method search] you can find the pattern within the given text. If a pattern is found, [RegExMatch] is returned and you can retrieve details of the results using functions such as [method RegExMatch.get_string] and [method RegExMatch.get_start].
 		[codeblock]
 		var regex = RegEx.new()
@@ -35,6 +35,8 @@
 		# Would print 01 03 3f 42
 		# Note that d0c would not match
 		[/codeblock]
+		[b]Note:[/b] Godot's regex implementation is based on the [url=https://www.pcre.org/]PCRE2[/url] library. You can view the full pattern reference [url=https://www.pcre.org/current/doc/html/pcre2pattern.html]here[/url].
+		[b]Tip:[/b] You can use [url=https://regexr.com/]Regexr[/url] to test regular expressions online.
 	</description>
 	<tutorials>
 	</tutorials>