diff --git a/tests/syntax-tests/highlighted/F#/string.fs b/tests/syntax-tests/highlighted/F#/string.fs
new file mode 100644
index 00000000..338b5d60
--- /dev/null
+++ b/tests/syntax-tests/highlighted/F#/string.fs
@@ -0,0 +1,182 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace Microsoft.FSharp.Core
+
+    open System
+    open System.Text
+    open Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicOperators
+    open Microsoft.FSharp.Core.Operators
+    open Microsoft.FSharp.Core.Operators.Checked
+    open Microsoft.FSharp.Collections
+    open Microsoft.FSharp.Primitives.Basics
+
+    [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
+    [<RequireQualifiedAccess>]
+    module String =
+        [<Literal>]
+        /// LOH threshold is calculated from Internal.Utilities.Library.LOH_SIZE_THRESHOLD_BYTES,
+        /// and is equal to 80_000 / sizeof<char>
+        let LOH_CHAR_THRESHOLD = 40_000
+
+        [<CompiledName("Length")>]
+        let length (str:string) = if isNull str then 0 else str.Length
+
+        [<CompiledName("Concat")>]
+        let concat sep (strings : seq<string>) =  
+
+            let concatArray sep (strings: string []) =
+                match length sep with
+                | 0 -> String.Concat strings
+                // following line should be used when this overload becomes part of .NET Standard (it's only in .NET Core)
+                //| 1 -> String.Join(sep.[0], strings, 0, strings.Length)
+                | _ -> String.Join(sep, strings, 0, strings.Length)
+
+            match strings with
+            | :? array<string> as arr -> 
+                concatArray sep arr
+
+            | :? list<string> as lst -> 
+                lst 
+                |> List.toArray 
+                |> concatArray sep
+
+            | _ ->
+                String.Join(sep, strings)
+
+        [<CompiledName("Iterate")>]
+        let iter (action : (char -> unit)) (str:string) =
+            if not (String.IsNullOrEmpty str) then
+                for i = 0 to str.Length - 1 do
+                    action str.[i] 
+
+        [<CompiledName("IterateIndexed")>]
+        let iteri action (str:string) =
+            if not (String.IsNullOrEmpty str) then
+                let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt(action)
+                for i = 0 to str.Length - 1 do
+                    f.Invoke(i, str.[i]) 
+
+        [<CompiledName("Map")>]
+        let map (mapping: char -> char) (str:string) =
+            if String.IsNullOrEmpty str then
+                String.Empty
+            else
+                let result = str.ToCharArray()
+                let mutable i = 0
+                for c in result do
+                    result.[i] <- mapping c
+                    i <- i + 1
+
+                new String(result)
+
+        [<CompiledName("MapIndexed")>]
+        let mapi (mapping: int -> char -> char) (str:string) =
+            let len = length str
+            if len = 0 then 
+                String.Empty
+            else
+                let result = str.ToCharArray()
+                let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt(mapping)
+
+                let mutable i = 0
+                while i < len do
+                    result.[i] <- f.Invoke(i, result.[i])
+                    i <- i + 1
+
+                new String(result)
+
+        [<CompiledName("Filter")>]
+        let filter (predicate: char -> bool) (str:string) =
+            let len = length str
+
+            if len = 0 then 
+                String.Empty
+
+            elif len > LOH_CHAR_THRESHOLD then
+                // By using SB here, which is twice slower than the optimized path, we prevent LOH allocations 
+                // and 'stop the world' collections if the filtering results in smaller strings.
+                // We also don't pre-allocate SB here, to allow for less mem pressure when filter result is small.
+                let res = StringBuilder()
+                str |> iter (fun c -> if predicate c then res.Append c |> ignore)
+                res.ToString()
+
+            else
+                // Must do it this way, since array.fs is not yet in scope, but this is safe
+                let target = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked len
+                let mutable i = 0
+                for c in str do
+                    if predicate c then 
+                        target.[i] <- c
+                        i <- i + 1
+
+                String(target, 0, i)
+
+        [<CompiledName("Collect")>]
+        let collect (mapping: char -> string) (str:string) =
+            if String.IsNullOrEmpty str then
+                String.Empty
+            else
+                let res = StringBuilder str.Length
+                str |> iter (fun c -> res.Append(mapping c) |> ignore)
+                res.ToString()
+
+        [<CompiledName("Initialize")>]
+        let init (count:int) (initializer: int-> string) =
+            if count < 0 then invalidArgInputMustBeNonNegative "count" count
+            let res = StringBuilder count
+            for i = 0 to count - 1 do 
+               res.Append(initializer i) |> ignore
+            res.ToString()
+
+        [<CompiledName("Replicate")>]
+        let replicate (count:int) (str:string) =
+            if count < 0 then invalidArgInputMustBeNonNegative "count" count
+
+            let len = length str
+            if len = 0 || count = 0 then 
+                String.Empty
+
+            elif len = 1 then
+                new String(str.[0], count)
+
+            elif count <= 4 then
+                match count with
+                | 1 -> str
+                | 2 -> String.Concat(str, str)
+                | 3 -> String.Concat(str, str, str)
+                | _ -> String.Concat(str, str, str, str)
+
+            else
+                // Using the primitive, because array.fs is not yet in scope. It's safe: both len and count are positive.
+                let target = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked (len * count)
+                let source = str.ToCharArray()
+
+                // O(log(n)) performance loop:
+                // Copy first string, then keep copying what we already copied 
+                // (i.e., doubling it) until we reach or pass the halfway point
+                Array.Copy(source, 0, target, 0, len)
+                let mutable i = len
+                while i * 2 < target.Length do
+                    Array.Copy(target, 0, target, i, i)
+                    i <- i * 2
+
+                // finally, copy the remain half, or less-then half
+                Array.Copy(target, 0, target, i, target.Length - i)
+                new String(target)
+
+
+        [<CompiledName("ForAll")>]
+        let forall predicate (str:string) =
+            if String.IsNullOrEmpty str then
+                true
+            else
+                let rec check i = (i >= str.Length) || (predicate str.[i] && check (i+1)) 
+                check 0
+
+        [<CompiledName("Exists")>]
+        let exists predicate (str:string) =
+            if String.IsNullOrEmpty str then
+                false
+            else
+                let rec check i = (i < str.Length) && (predicate str.[i] || check (i+1)) 
+                check 0  
diff --git a/tests/syntax-tests/source/F#/LICENSE.md b/tests/syntax-tests/source/F#/LICENSE.md
new file mode 100644
index 00000000..7473c821
--- /dev/null
+++ b/tests/syntax-tests/source/F#/LICENSE.md
@@ -0,0 +1,27 @@
+The `string.fs` file has been added from https://github.com/dotnet/fsharp under the following license:
+
+```text
+The MIT License (MIT)
+
+Copyright (c) Microsoft Corporation.
+All rights reserved.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+```
diff --git a/tests/syntax-tests/source/F#/string.fs b/tests/syntax-tests/source/F#/string.fs
new file mode 100644
index 00000000..37e8fcd7
--- /dev/null
+++ b/tests/syntax-tests/source/F#/string.fs
@@ -0,0 +1,182 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace Microsoft.FSharp.Core
+
+    open System
+    open System.Text
+    open Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicOperators
+    open Microsoft.FSharp.Core.Operators
+    open Microsoft.FSharp.Core.Operators.Checked
+    open Microsoft.FSharp.Collections
+    open Microsoft.FSharp.Primitives.Basics
+
+    [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
+    [<RequireQualifiedAccess>]
+    module String =
+        [<Literal>]
+        /// LOH threshold is calculated from Internal.Utilities.Library.LOH_SIZE_THRESHOLD_BYTES,
+        /// and is equal to 80_000 / sizeof<char>
+        let LOH_CHAR_THRESHOLD = 40_000
+
+        [<CompiledName("Length")>]
+        let length (str:string) = if isNull str then 0 else str.Length
+
+        [<CompiledName("Concat")>]
+        let concat sep (strings : seq<string>) =  
+
+            let concatArray sep (strings: string []) =
+                match length sep with
+                | 0 -> String.Concat strings
+                // following line should be used when this overload becomes part of .NET Standard (it's only in .NET Core)
+                //| 1 -> String.Join(sep.[0], strings, 0, strings.Length)
+                | _ -> String.Join(sep, strings, 0, strings.Length)
+
+            match strings with
+            | :? array<string> as arr -> 
+                concatArray sep arr
+
+            | :? list<string> as lst -> 
+                lst 
+                |> List.toArray 
+                |> concatArray sep
+
+            | _ ->
+                String.Join(sep, strings)
+
+        [<CompiledName("Iterate")>]
+        let iter (action : (char -> unit)) (str:string) =
+            if not (String.IsNullOrEmpty str) then
+                for i = 0 to str.Length - 1 do
+                    action str.[i] 
+
+        [<CompiledName("IterateIndexed")>]
+        let iteri action (str:string) =
+            if not (String.IsNullOrEmpty str) then
+                let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt(action)
+                for i = 0 to str.Length - 1 do
+                    f.Invoke(i, str.[i]) 
+
+        [<CompiledName("Map")>]
+        let map (mapping: char -> char) (str:string) =
+            if String.IsNullOrEmpty str then
+                String.Empty
+            else
+                let result = str.ToCharArray()
+                let mutable i = 0
+                for c in result do
+                    result.[i] <- mapping c
+                    i <- i + 1
+
+                new String(result)
+
+        [<CompiledName("MapIndexed")>]
+        let mapi (mapping: int -> char -> char) (str:string) =
+            let len = length str
+            if len = 0 then 
+                String.Empty
+            else
+                let result = str.ToCharArray()
+                let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt(mapping)
+
+                let mutable i = 0
+                while i < len do
+                    result.[i] <- f.Invoke(i, result.[i])
+                    i <- i + 1
+
+                new String(result)
+
+        [<CompiledName("Filter")>]
+        let filter (predicate: char -> bool) (str:string) =
+            let len = length str
+
+            if len = 0 then 
+                String.Empty
+
+            elif len > LOH_CHAR_THRESHOLD then
+                // By using SB here, which is twice slower than the optimized path, we prevent LOH allocations 
+                // and 'stop the world' collections if the filtering results in smaller strings.
+                // We also don't pre-allocate SB here, to allow for less mem pressure when filter result is small.
+                let res = StringBuilder()
+                str |> iter (fun c -> if predicate c then res.Append c |> ignore)
+                res.ToString()
+
+            else
+                // Must do it this way, since array.fs is not yet in scope, but this is safe
+                let target = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked len
+                let mutable i = 0
+                for c in str do
+                    if predicate c then 
+                        target.[i] <- c
+                        i <- i + 1
+
+                String(target, 0, i)
+
+        [<CompiledName("Collect")>]
+        let collect (mapping: char -> string) (str:string) =
+            if String.IsNullOrEmpty str then
+                String.Empty
+            else
+                let res = StringBuilder str.Length
+                str |> iter (fun c -> res.Append(mapping c) |> ignore)
+                res.ToString()
+
+        [<CompiledName("Initialize")>]
+        let init (count:int) (initializer: int-> string) =
+            if count < 0 then invalidArgInputMustBeNonNegative "count" count
+            let res = StringBuilder count
+            for i = 0 to count - 1 do 
+               res.Append(initializer i) |> ignore
+            res.ToString()
+
+        [<CompiledName("Replicate")>]
+        let replicate (count:int) (str:string) =
+            if count < 0 then invalidArgInputMustBeNonNegative "count" count
+
+            let len = length str
+            if len = 0 || count = 0 then 
+                String.Empty
+
+            elif len = 1 then
+                new String(str.[0], count)
+
+            elif count <= 4 then
+                match count with
+                | 1 -> str
+                | 2 -> String.Concat(str, str)
+                | 3 -> String.Concat(str, str, str)
+                | _ -> String.Concat(str, str, str, str)
+
+            else
+                // Using the primitive, because array.fs is not yet in scope. It's safe: both len and count are positive.
+                let target = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked (len * count)
+                let source = str.ToCharArray()
+
+                // O(log(n)) performance loop:
+                // Copy first string, then keep copying what we already copied 
+                // (i.e., doubling it) until we reach or pass the halfway point
+                Array.Copy(source, 0, target, 0, len)
+                let mutable i = len
+                while i * 2 < target.Length do
+                    Array.Copy(target, 0, target, i, i)
+                    i <- i * 2
+
+                // finally, copy the remain half, or less-then half
+                Array.Copy(target, 0, target, i, target.Length - i)
+                new String(target)
+
+
+        [<CompiledName("ForAll")>]
+        let forall predicate (str:string) =
+            if String.IsNullOrEmpty str then
+                true
+            else
+                let rec check i = (i >= str.Length) || (predicate str.[i] && check (i+1)) 
+                check 0
+
+        [<CompiledName("Exists")>]
+        let exists predicate (str:string) =
+            if String.IsNullOrEmpty str then
+                false
+            else
+                let rec check i = (i < str.Length) && (predicate str.[i] || check (i+1)) 
+                check 0