Create rule S6397: Character classes in regular expressions should not contain only one character (#648)

Co-authored-by: karim-ouerghemmi-sonarsource <karim-ouerghemmi-sonarsource@users.noreply.github.com>

rules/S6397/metadata.json

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+{
+}

rules/S6397/php/metadata.json

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+{
+  "title": "Character classes in regular expressions should not contain only one character",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "regex"
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-6397",
+  "sqKey": "S6397",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown"
+}

rules/S6397/php/rule.adoc

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+Character classes in regular expressions are a convenient way to match one of several possible characters by listing the allowed characters or ranges of characters. If a character class contains only one character, the effect is the same as just writing the character without a character class.
+
+Thus, having only one character in a character class is usually a simple oversight that remained after removing other characters of the class.
+
+== Noncompliant Code Example
+
+----
+/a[b]c/
+----
+
+== Compliant Solution
+
+----
+/abc/
+----
+
+== Exceptions
+
+This rule does not raise when the character inside the class is a metacharacter. This notation is sometimes used to avoid escaping (e.g., ``++[.]{3}++`` to match three dots).   
+
+ifdef::env-github,rspecator-view[]
+
+'''
+== Implementation Specification
+(visible only on this page)
+
+=== Message
+
+Replace this character class by the character itself.
+
+=== Highlighting
+
+The character class.
+
+endif::env-github,rspecator-view[]