Implementation
C#

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Other implementations
var
  i: integer;
begin
  t := '';
  for i := 1 to length(s) do
    if s[i] < #128 then t := t + s[i];
end.
uses RegExpr;
var
  s,t: string;
begin
  t := ReplaceRegExpr('[^\u0000-\u007F]+', s, '', False);
end.
t = s.gsub(/[^[:ascii:]]/, "")
import "regexp"
re := regexp.MustCompile("[[:^ascii:]]")
t := re.ReplaceAllLiteralString(s, "")
t = s.gsub(/[[:^ascii:]]/, "")
import std.array;
import std.algorithm.iteration;
import std.ascii;
auto t = s.filter!(a => a.isASCII).array;
import re
t = re.sub('[^\u0000-\u007f]', '',  s)
import Data.Char
f = filter isAscii
t = f s
import Data.Char
t = filter isAscii s
let t = s.replace(|c: char| !c.is_ascii(), "");
$t = preg_replace('/[^[:ascii:]]/', '', $s);