diff --git a/docs/v1.9/reference/composition.md b/docs/v1.9/reference/composition.md
index d264cf2e..e3a352a2 100644
--- a/docs/v1.9/reference/composition.md
+++ b/docs/v1.9/reference/composition.md
@@ -526,6 +526,7 @@ Currently only `multiply` is supported.
 * string transform type `Convert`, accepts one of `ToUpper`, `ToLower`, `ToBase64`, `FromBase64`.
 * string transform type `TrimPrefix`, accepts a string to be trimmed from the beginning of the input.
 * string transform type `TrimSuffix`, accepts a string to be trimmed from the end of the input.
+* string transform type `Regexp`, accepts a string for regexp to be applied to.
 
 ```yaml
 # If you omit the field type, by default type is set to `Format` 
@@ -583,6 +584,15 @@ Currently only `multiply` is supported.
   string:
      type: TrimSuffix
      trim: '-test'
+
+# If the value of the 'from' field is 'arn:aws:iam::42:example, the value of the
+# 'to' field will be set to "42". Note that the 'to' field is always a string. 
+- type: string
+  string:
+     type: Regexp
+     regexp:
+      match: 'arn:aws:iam::(\d+):.*'
+      group: 1  # Optional capture group. Omit to match the entire regexp.
 ```
 
 `convert`. Transforms values of one type to another, for example from a string