Change variable names for clarity in regex examples (#6424)

This update improves the variable naming in the Dart regex example for
accurate understanding:

- Renamed `numbers` → `digitSequence`.
- Renamed `allCharacters` → `lettersOnly` .

This changes are made in two directories:

1) `/examples/misc/test/library_tour/core_test.dart`
2) `/src/content/libraries/dart-core.md`

This ensures and follows the contributing guidelines.
As per the wish, the variables name are modified for greater understanding.

Fixes #6410

Author: J-Manoj-06

examples/misc/test/library_tour/core_test.dart

@@ -166,32 +166,32 @@ void main() {
     test('RegExp', () {
       // #docregion regexp
       // Here's a regular expression for one or more digits.
-      var numbers = RegExp(r'\d+');
+      var digitSequence = RegExp(r'\d+');
 
-      var allCharacters = 'llamas live fifteen to twenty years';
+      var lettersOnly = 'llamas live fifteen to twenty years';
       var someDigits = 'llamas live 15 to 20 years';
 
       // contains() can use a regular expression.
-      assert(!allCharacters.contains(numbers));
-      assert(someDigits.contains(numbers));
+      assert(!lettersOnly.contains(digitSequence));
+      assert(someDigits.contains(digitSequence));
 
       // Replace every match with another string.
-      var exedOut = someDigits.replaceAll(numbers, 'XX');
+      var exedOut = someDigits.replaceAll(digitSequence, 'XX');
       assert(exedOut == 'llamas live XX to XX years');
       // #enddocregion regexp
     });
 
     test('match', () {
       void testMatch() {
         // #docregion match
-        var numbers = RegExp(r'\d+');
+        var digitSequence = RegExp(r'\d+');
         var someDigits = 'llamas live 15 to 20 years';
 
         // Check whether the reg exp has a match in a string.
-        assert(numbers.hasMatch(someDigits));
+        assert(digitSequence.hasMatch(someDigits));
 
         // Loop through all matches.
-        for (final match in numbers.allMatches(someDigits)) {
+        for (final match in digitSequence.allMatches(someDigits)) {
           print(match.group(0)); // 15, then 20
         }
         // #enddocregion match

src/content/libraries/dart-core.md

@@ -255,17 +255,17 @@ matching of strings.
 <?code-excerpt "misc/test/library_tour/core_test.dart (regexp)"?>
 ```dart
 // Here's a regular expression for one or more digits.
-var numbers = RegExp(r'\d+');
+var digitSequence = RegExp(r'\d+');
 
-var allCharacters = 'llamas live fifteen to twenty years';
+var lettersOnly = 'llamas live fifteen to twenty years';
 var someDigits = 'llamas live 15 to 20 years';
 
 // contains() can use a regular expression.
-assert(!allCharacters.contains(numbers));
-assert(someDigits.contains(numbers));
+assert(!lettersOnly.contains(digitSequence));
+assert(someDigits.contains(digitSequence));
 
 // Replace every match with another string.
-var exedOut = someDigits.replaceAll(numbers, 'XX');
+var exedOut = someDigits.replaceAll(digitSequence, 'XX');
 assert(exedOut == 'llamas live XX to XX years');
 ```
 
@@ -274,14 +274,14 @@ provides access to a regular expression match.
 
 <?code-excerpt "misc/test/library_tour/core_test.dart (match)"?>
 ```dart
-var numbers = RegExp(r'\d+');
+var digitSequence = RegExp(r'\d+');
 var someDigits = 'llamas live 15 to 20 years';
 
 // Check whether the reg exp has a match in a string.
-assert(numbers.hasMatch(someDigits));
+assert(digitSequence.hasMatch(someDigits));
 
 // Loop through all matches.
-for (final match in numbers.allMatches(someDigits)) {
+for (final match in digitSequence.allMatches(someDigits)) {
   print(match.group(0)); // 15, then 20
 }
 ```