diff --git a/reference/reference.go b/reference/reference.go
index 4723c3ec..ab15f1e6 100644
--- a/reference/reference.go
+++ b/reference/reference.go
@@ -19,6 +19,9 @@
 //	digest-algorithm-separator      := /[+.-_]/
 //	digest-algorithm-component      := /[A-Za-z][A-Za-z0-9]*/
 //	digest-hex                      := /[0-9a-fA-F]{32,}/ ; At least 128 bit digest value
+//
+//      identifier                      := /[a-f0-9]{64}/
+//      short-identifier                := /[a-f0-9]{6,64}/
 package reference
 
 import (
diff --git a/reference/regexp.go b/reference/regexp.go
index 5ba4de85..405e995d 100644
--- a/reference/regexp.go
+++ b/reference/regexp.go
@@ -68,6 +68,25 @@ var (
 	ReferenceRegexp = anchored(capture(NameRegexp),
 		optional(literal(":"), capture(TagRegexp)),
 		optional(literal("@"), capture(DigestRegexp)))
+
+	// IdentifierRegexp is the format for string identifier used as a
+	// content addressable identifier using sha256. These identifiers
+	// are like digests without the algorithm, since sha256 is used.
+	IdentifierRegexp = match(`([a-f0-9]{64})`)
+
+	// ShortIdentifierRegexp is the format used to represent a prefix
+	// of an identifier. A prefix may be used to match a sha256 identifier
+	// within a list of trusted identifiers.
+	ShortIdentifierRegexp = match(`([a-f0-9]{6,64})`)
+
+	// anchoredIdentifierRegexp is used to check or match an
+	// identifier value, anchored at start and end of string.
+	anchoredIdentifierRegexp = anchored(IdentifierRegexp)
+
+	// anchoredShortIdentifierRegexp is used to check if a value
+	// is a possible identifier prefix, anchored at start and end
+	// of string.
+	anchoredShortIdentifierRegexp = anchored(ShortIdentifierRegexp)
 )
 
 // match compiles the string to a regular expression.
diff --git a/reference/regexp_test.go b/reference/regexp_test.go
index aef39bb1..c2126399 100644
--- a/reference/regexp_test.go
+++ b/reference/regexp_test.go
@@ -487,3 +487,67 @@ func TestReferenceRegexp(t *testing.T) {
 	}
 
 }
+
+func TestIdentifierRegexp(t *testing.T) {
+	fullCases := []regexpMatch{
+		{
+			input: "da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf9821",
+			match: true,
+		},
+		{
+			input: "7EC43B381E5AEFE6E04EFB0B3F0693FF2A4A50652D64AEC573905F2DB5889A1C",
+			match: false,
+		},
+		{
+			input: "da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf",
+			match: false,
+		},
+		{
+			input: "sha256:da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf9821",
+			match: false,
+		},
+		{
+			input: "da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf98218482",
+			match: false,
+		},
+	}
+
+	shortCases := []regexpMatch{
+		{
+			input: "da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf9821",
+			match: true,
+		},
+		{
+			input: "7EC43B381E5AEFE6E04EFB0B3F0693FF2A4A50652D64AEC573905F2DB5889A1C",
+			match: false,
+		},
+		{
+			input: "da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf",
+			match: true,
+		},
+		{
+			input: "sha256:da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf9821",
+			match: false,
+		},
+		{
+			input: "da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf98218482",
+			match: false,
+		},
+		{
+			input: "da304",
+			match: false,
+		},
+		{
+			input: "da304e",
+			match: true,
+		},
+	}
+
+	for i := range fullCases {
+		checkRegexp(t, anchoredIdentifierRegexp, fullCases[i])
+	}
+
+	for i := range shortCases {
+		checkRegexp(t, anchoredShortIdentifierRegexp, shortCases[i])
+	}
+}