Deprecated: Improve $.trim performance for strings with lots of whitespace

Regex imp implementation takes `O(N^2)` time to trim the string when
multiple adjacent spaces were present.

The new expression require that the "whitespace run" starts from
a non-whitespace to avoid `O(N^2)` behavior when the engine would
try matching `\s+$` at each space position.

Closes gh-461
Ref jquery/jquery#5068

Author: vlsi

src/jquery/core.js

@@ -15,9 +15,9 @@ var findProp,
 	rattrHashTest = /\[(\s*[-\w]+\s*)([~|^$*]?=)\s*([-\w#]*?#[-\w#]*)\s*\]/,
 	rattrHashGlob = /\[(\s*[-\w]+\s*)([~|^$*]?=)\s*([-\w#]*?#[-\w#]*)\s*\]/g,
 
-	// Support: Android <=4.0 only
-	// Make sure we trim BOM and NBSP
-	rtrim = /^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g;
+	// Require that the "whitespace run" starts from a non-whitespace
+	// to avoid O(N^2) behavior when the engine would try matching "\s+$" at each space position.
+	rtrim = /^[\s\uFEFF\xA0]+|([^\s\uFEFF\xA0])[\s\uFEFF\xA0]+$/g;
 
 migratePatchFunc( jQuery.fn, "init", function( arg1 ) {
 	var args = Array.prototype.slice.call( arguments );
@@ -109,7 +109,7 @@ if ( jQueryVersionSince( "3.1.1" ) ) {
 	migratePatchAndWarnFunc( jQuery, "trim", function( text ) {
 		return text == null ?
 			"" :
-			( text + "" ).replace( rtrim, "" );
+			( text + "" ).replace( rtrim, "$1" );
 	}, "trim",
 	"jQuery.trim is deprecated; use String.prototype.trim" );
 }