• C#

var t = string.Concat(s.Where(c => char.IsDigit(c)));

• Clojure

(let [s "1a22b3c4de5f6"
      t (str/replace s #"[^\d]" "")]
  (println t))

• Dart

var t = String.fromCharCodes(
	s.codeUnits.where((x) => (x ^ 0x30) <= 9) );

• Go

re := regexp.MustCompile("[^\\d]")
t := re.ReplaceAllLiteralString(s, "")

• Haskell

t = filter (`elem` ['0'..'9']) s

• JS

let t = s.replaceAll(/\D/g, '')

• JS

t = s.replace(/[^\d]/gm,"");

• Java

String t = s.replaceAll("\\D+", "");

• Java

String t = s.chars()
    .filter(Character::isDigit)
    .mapToObj(x -> valueOf((char) x))
    .collect(joining());

• Java

String t = "";
char c;
int i, n = s.length();
for (i = 0; i < n; ++i)
    if (isDigit(c = s.charAt(i)))
        t = t + c;

• Java

String t = "";
for (char c : s.toCharArray())
    if (isDigit(c)) t = t + c;

• Java

StringBuilder tb = new StringBuilder();
for (int i=0; i<s.length(); i++){
    if(Character.isDigit(s.charAt(i)))
        tb.append(s.charAt(i));
}
String t = tb.toString();

• Lisp

(let ((*t* (remove-if-not #'digit-char-p *s*)))
	(format t "~A~%" *t*))

• Pascal

for i := 1 to length(s) do
    if s[i] in ['0'..'9'] then
        t := t + s[i];

• Pascal

t := ReplaceRegExpr('[^\d]', s, ''); 

• Perl

($t = $s) =~ tr/0-9//cd;

• Perl

my $t = $s;
$t =~ s/\D+//g;

• Python

t = re.sub(r"\D", "", s)

• Python

t = ''.join(x for x in s if x in digits)

• Ruby

t = s.delete("^0-9")

• Rust

let t: String = s.chars().filter(|c| c.is_digit(10)).collect();