diff --git a/spec/std/fiber/execution_context/global_queue_spec.cr b/spec/std/fiber/execution_context/global_queue_spec.cr
index b6b866b79e88..cc409c45efc6 100644
--- a/spec/std/fiber/execution_context/global_queue_spec.cr
+++ b/spec/std/fiber/execution_context/global_queue_spec.cr
@@ -107,7 +107,7 @@ describe Fiber::ExecutionContext::GlobalQueue do
 
           loop do
             if fiber = queue.pop?
-              fc = fibers.find { |x| x.@fiber == fiber }.not_nil!
+              fc = fibers.find! { |x| x.@fiber == fiber }
               queue.push(fiber) if fc.increment < increments
               slept = 0
             elsif slept < 100
@@ -163,7 +163,7 @@ describe Fiber::ExecutionContext::GlobalQueue do
           }
 
           execute = ->(fiber : Fiber) {
-            fc = fibers.find { |x| x.@fiber == fiber }.not_nil!
+            fc = fibers.find! { |x| x.@fiber == fiber }
 
             if fc.increment < increments
               batch.push(fc.@fiber)
diff --git a/spec/std/fiber/execution_context/runnables_spec.cr b/spec/std/fiber/execution_context/runnables_spec.cr
index 0df82f3f5bda..3930bf2f07a6 100644
--- a/spec/std/fiber/execution_context/runnables_spec.cr
+++ b/spec/std/fiber/execution_context/runnables_spec.cr
@@ -202,7 +202,7 @@ describe Fiber::ExecutionContext::Runnables do
           slept = 0
 
           execute = ->(fiber : Fiber) {
-            fc = fibers.find { |x| x.@fiber == fiber }.not_nil!
+            fc = fibers.find! { |x| x.@fiber == fiber }
             runnables.push(fiber) if fc.increment < increments
           }
 
diff --git a/spec/std/regex_spec.cr b/spec/std/regex_spec.cr
index 230976d6ad3e..b2220959fdb4 100644
--- a/spec/std/regex_spec.cr
+++ b/spec/std/regex_spec.cr
@@ -547,8 +547,8 @@ describe "Regex" do
   describe ".union" do
     it "constructs a Regex that matches things any of its arguments match" do
       re = Regex.union(/skiing/i, "sledding")
-      re.match("Skiing").not_nil![0].should eq "Skiing"
-      re.match("sledding").not_nil![0].should eq "sledding"
+      re.match!("Skiing")[0].should eq "Skiing"
+      re.match!("sledding")[0].should eq "sledding"
     end
 
     it "returns a regular expression that will match passed arguments" do
diff --git a/spec/std/string_spec.cr b/spec/std/string_spec.cr
index 2bd5574dbc2a..a80ed3a50bfe 100644
--- a/spec/std/string_spec.cr
+++ b/spec/std/string_spec.cr
@@ -2417,18 +2417,26 @@ describe "String" do
     end
   end
 
-  it "has match" do
-    "FooBar".match(/oo/).not_nil![0].should eq("oo")
-  end
+  describe "#match" do
+    it "has match" do
+      match = "FooBar".match(/oo/).should_not be_nil
+      match[0].should eq("oo")
+    end
 
-  it "matches with position" do
-    "こんにちは".match(/./, 1).not_nil![0].should eq("ん")
-  end
+    it "matches with position" do
+      match = "こんにちは".match(/./, 1).should_not be_nil
+      match[0].should eq("ん")
+    end
 
-  it "matches empty string" do
-    match = "".match(/.*/).not_nil!
-    match.group_size.should eq(0)
-    match[0].should eq("")
+    it "matches empty string" do
+      match = "".match(/.*/).should_not be_nil
+      match.group_size.should eq(0)
+      match[0].should eq("")
+    end
+
+    it "returns nil" do
+      "foo".match(/bar/).should be_nil
+    end
   end
 
   it "matches, but returns Bool" do
diff --git a/src/compiler/crystal/compiler.cr b/src/compiler/crystal/compiler.cr
index cebd5d222a5c..0d275fe796a0 100644
--- a/src/compiler/crystal/compiler.cr
+++ b/src/compiler/crystal/compiler.cr
@@ -699,7 +699,7 @@ module Crystal
         if @progress_tracker.stats?
           if result["reused"].as_bool
             name = result["name"].as_s
-            unit = units.find { |unit| unit.name == name }.not_nil!
+            unit = units.find! { |unit| unit.name == name }
             unit.reused_previous_compilation = true
           end
         end
diff --git a/src/compiler/crystal/crystal_path.cr b/src/compiler/crystal/crystal_path.cr
index e957a513a714..dbee841e9268 100644
--- a/src/compiler/crystal/crystal_path.cr
+++ b/src/compiler/crystal/crystal_path.cr
@@ -106,7 +106,7 @@ module Crystal
 
       # Check if it's a wildcard.
       if filename.ends_with?("/*") || (recursive = filename.ends_with?("/**"))
-        filename_dir_index = filename.rindex('/').not_nil!
+        filename_dir_index = filename.rindex!('/')
         filename_dir = filename[0..filename_dir_index]
         relative_dir = "#{relative_to}/#{filename_dir}"
         if File.exists?(relative_dir)
diff --git a/src/compiler/crystal/tools/formatter.cr b/src/compiler/crystal/tools/formatter.cr
index dcd7be66df08..2070327e4f90 100644
--- a/src/compiler/crystal/tools/formatter.cr
+++ b/src/compiler/crystal/tools/formatter.cr
@@ -5111,7 +5111,7 @@ module Crystal
         next if doc_comment.start_line > doc_comment.end_line
 
         first_line = lines[doc_comment.start_line]
-        sharp_index = first_line.index('#').not_nil!
+        sharp_index = first_line.index!('#')
 
         comment = String.build do |str|
           (doc_comment.start_line..doc_comment.end_line).each do |i|
diff --git a/src/compiler/crystal/tools/playground/server.cr b/src/compiler/crystal/tools/playground/server.cr
index 1729bd7a98b3..8d36d486c40e 100644
--- a/src/compiler/crystal/tools/playground/server.cr
+++ b/src/compiler/crystal/tools/playground/server.cr
@@ -453,7 +453,7 @@ module Crystal::Playground
       public_dir = File.join(playground_dir, "public")
 
       agent_ws = PathWebSocketHandler.new "/agent" do |ws, context|
-        match_data = context.request.path.not_nil!.match(/\/(\d+)\/(\d+)$/).not_nil!
+        match_data = context.request.path.not_nil!.match!(/\/(\d+)\/(\d+)$/)
         session_key = match_data[1].to_i
         tag = match_data[2].to_i
         Log.info { "#{context.request.path} WebSocket connected (session=#{session_key}, tag=#{tag})" }