diff --git a/ascii.libsonnet b/ascii.libsonnet
index f431376..05c2f38 100644
--- a/ascii.libsonnet
+++ b/ascii.libsonnet
@@ -96,4 +96,37 @@ local d = import 'github.com/jsonnet-libs/docsonnet/doc-util/main.libsonnet';
       fraction(expectFraction),
       exponent(expectExponent),
     ]),
+
+  '#stringToRFC1123': d.fn(
+    |||
+      `stringToRFC113` converts a strings to match RFC1123, replacing non-alphanumeric characters with dashes. It'll throw an assertion if the string is too long.
+
+      * RFC 1123. This means the string must:
+      * - contain at most 63 characters
+      * - contain only lowercase alphanumeric characters or '-'
+      * - start with an alphanumeric character
+      * - end with an alphanumeric character
+    |||,
+    [d.arg('str', d.T.string)]
+  ),
+  stringToRFC1123(str):
+    // lowercase alphabetic characters
+    local lowercase = std.asciiLower(str);
+    // replace non-alphanumeric characters with dashes
+    local alphanumeric =
+      std.join(
+        '',
+        std.map(
+          function(c)
+            if self.isLower(c)
+               || self.isNumber(c)
+            then c
+            else '-',
+          std.stringChars(lowercase)
+        )
+      );
+    // remove leading/trailing dashes
+    local return = std.stripChars(alphanumeric, '-');
+    assert std.length(return) <= 63 : 'String too long';
+    return,
 }
diff --git a/docs/ascii.md b/docs/ascii.md
index bee1eb3..95382d5 100644
--- a/docs/ascii.md
+++ b/docs/ascii.md
@@ -17,6 +17,7 @@ local ascii = import "github.com/jsonnet-libs/xtd/ascii.libsonnet"
 * [`fn isStringJSONNumeric(str)`](#fn-isstringjsonnumeric)
 * [`fn isStringNumeric(str)`](#fn-isstringnumeric)
 * [`fn isUpper(c)`](#fn-isupper)
+* [`fn stringToRFC1123(str)`](#fn-stringtorfc1123)
 
 ## Fields
 
@@ -58,4 +59,18 @@ isStringNumeric(str)
 isUpper(c)
 ```
 
-`isUpper` reports whether ASCII character `c` is a upper case letter
\ No newline at end of file
+`isUpper` reports whether ASCII character `c` is a upper case letter
+
+### fn stringToRFC1123
+
+```ts
+stringToRFC1123(str)
+```
+
+`stringToRFC113` converts a strings to match RFC1123, replacing non-alphanumeric characters with dashes. It'll throw an assertion if the string is too long.
+
+* RFC 1123. This means the string must:
+* - contain at most 63 characters
+* - contain only lowercase alphanumeric characters or '-'
+* - start with an alphanumeric character
+* - end with an alphanumeric character