That day when regex can be faster than code

CHANGELOG.md

@@ -10,10 +10,12 @@
 
 #### Enhancements
 
-* None.
-
 #### Bug Fixes
 
+* Fix false positive for LetVarWhitespaceRule.  
+  [PaulTaykalo](https://github.com/PaulTaykalo)
+  [#2956](https://github.com/realm/SwiftLint/issues/2956)
+
 * Fix for false-positive identical operands rule.  
   [PaulTaykalo](https://github.com/PaulTaykalo)
   [#2953](https://github.com/realm/SwiftLint/issues/2953)

Source/SwiftLintFramework/Rules/Style/LetVarWhitespaceRule.swift

@@ -196,10 +196,9 @@ public struct LetVarWhitespaceRule: ConfigurationProviderRule, OptInRule, Automa
             }
         }
 
-        let directives: Set = ["#if", "#elseif", "#else", "#endif", "#!", "#warning", "#error"]
         let directiveLines = file.lines.filter {
-            let trimmed = $0.content.trimmingCharacters(in: .whitespaces)
-            return directives.contains(where: trimmed.hasPrefix)
+            return regex(#"^\s*#(if|elseif|else|endif|\!|warning|error)"#)
+                .firstMatch(in: $0.content, options: [], range: $0.content.fullNSRange) != nil
         }
 
         result.formUnion(directiveLines.map { $0.index - 1 })