Separate string by thousands (regex version)

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let separateByThousands (input: string) =
    let regex = System.Text.RegularExpressions.Regex (@"(\d)(?=(\d{3})+(?!\d))")
    regex.Replace (input, "$1 ")

val separateByThousands : input:string -> string

val input : string

Multiple items
val string : value:'T -> string

--------------------
type string = System.String

val regex : System.Text.RegularExpressions.Regex

namespace System

namespace System.Text

namespace System.Text.RegularExpressions

Multiple items
type Regex =
  new : pattern:string -> Regex + 2 overloads
  member GetGroupNames : unit -> string[]
  member GetGroupNumbers : unit -> int[]
  member GroupNameFromNumber : i:int -> string
  member GroupNumberFromName : name:string -> int
  member IsMatch : input:string -> bool + 1 overload
  member Match : input:string -> Match + 2 overloads
  member MatchTimeout : TimeSpan
  member Matches : input:string -> MatchCollection + 1 overload
  member Options : RegexOptions
  ...

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System.Text.RegularExpressions.Regex(pattern: string) : System.Text.RegularExpressions.Regex
System.Text.RegularExpressions.Regex(pattern: string, options: System.Text.RegularExpressions.RegexOptions) : System.Text.RegularExpressions.Regex
System.Text.RegularExpressions.Regex(pattern: string, options: System.Text.RegularExpressions.RegexOptions, matchTimeout: System.TimeSpan) : System.Text.RegularExpressions.Regex

System.Text.RegularExpressions.Regex.Replace(input: string, evaluator: System.Text.RegularExpressions.MatchEvaluator) : string
System.Text.RegularExpressions.Regex.Replace(input: string, replacement: string) : string
System.Text.RegularExpressions.Regex.Replace(input: string, evaluator: System.Text.RegularExpressions.MatchEvaluator, count: int) : string
System.Text.RegularExpressions.Regex.Replace(input: string, replacement: string, count: int) : string
System.Text.RegularExpressions.Regex.Replace(input: string, evaluator: System.Text.RegularExpressions.MatchEvaluator, count: int, startat: int) : string
System.Text.RegularExpressions.Regex.Replace(input: string, replacement: string, count: int, startat: int) : string