ISO/IEC 23736-6:2020

INTERNATIONAL ISO/IEC
STANDARD 23736-6
First edition
2020-02
Information technology — Digital
publishing — EPUB 3.0.1 —
Part 6:
Canonical fragment identifiers
Technologies de l'information — Publications numériques — EPUB
3.0.1 —
Partie 6: Identificateurs de fragment canoniques
Reference number
©
ISO/IEC 2020
© ISO/IEC 2020
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Published in Switzerland
ii © ISO/IEC 2020 – All rights reserved

Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are
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Attention is drawn to the possibility that some of the elements of this document may be the subject of
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see www.iso.org/iso/foreword.html.
This document was prepared by the World Wide Web Consortium (W3C) (as EPUB Canonical Fragment
Identifiers 1.1) and drafted in accordance with its editorial rules. It was adopted, under the JTC 1
PAS procedure, by Joint Technical Committee ISO/IEC JTC 1, Information technology.
A list of all parts in the ISO/IEC 23736 series can be found on the ISO websitte.e
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
© ISO/IEC 2020 – All rights reserved iii

EPUB Canonical Fragment Identifiers 1.1
Recommended Specification 5 January 2017
This version
http://www.idpf.org/epub/linking/cfi/epub-cfi-20170105.html
Latest version
http://www.idpf.org/epub/linking/cfi/epub-cfi.html
Previous version
http://www.idpf.org/epub/linking/cfi/epub-cfi-20161130.html
Previous recommendation
http://www.idpf.org/epub/linking/cfi/epub-cfi-20140628.html
Document history
Changes to this document
Issues addressed in this revision
Report an issue
Errata
Editors
Peter Sorotokin, Adobe
Garth Conboy, Google Inc.
Brady Duga, Google Inc.
John Rivlin, Google Inc.
Don Beaver, Apple Inc.
Kevin Ballard, Apple Inc.
Alastair Fettes, Apple Inc.
Daniel Weck, DAISY Consortium
All rights reserved. This work is protected under Title 17 of the United States Code. Reproduction and
dissemination of this work with changes is prohibited except with the written permission of the International
Digital Publishing Forum (IDPF).
EPUB is a registered trademark of the International Digital Publishing Forum.
Status of this Document
This section describes the status of this document at the time of its publication. Other
documents might supersede this document.
© ISO/IEC 2020 – All rights reserved 1

This document was produced by the EPUB Working Group under the EPUB Working
Group Charter approved on 8 July 2015.
This document has been reviewed by the IDPF membership and is endorsed by the IDPF
Board as a Recommended Specification. This document is considered stable and can be
referenced from other specifications and documents.
Feedback on this document can be provided to the EPUB Working Group's mailing list or
issue tracker.
This document is governed by the IDPF Policies and Procedures.
Table of Contents
1. Overview
1.1. Purpose and Scope
1.2. Terminology
1.3. Typographic Conventions
1.4. Conformance Statements
2. EPUB CFI Definition
2.1. Introduction
2.2. Syntax
2.3. Character Escaping
3. EPUB CFI Processing
3.1. Path Resolution
3.1.1. Step Reference to Child Element or Character Data (/)
3.1.2. XML ID Assertion ([)
3.1.3. Step Indirection (!)
3.1.4. Character Offset (:)
3.1.5. Temporal Offset (~)
3.1.6. Spatial Offset (@)
3.1.7. Temporal-Spatial Offset (~ + @)
3.1.8. Text Location Assertion ([)
3.1.9. Side Bias ([ + ;s=)
3.1.10. Examples
3.2. Sorting Rules
3.3. Intra-Publication CFIs
3.4. Simple Ranges
3.5. Intended Target Location Correction
4. Extending EPUB CFIs
References
› 1 Overview
› 1.1 Purpose and Scope
This section is informative
This specification, EPUB Canonical Fragment Identifier (epubcfi), defines a standardized
method for referencing arbitrary content within an EPUB® Publication through the use of
fragment identifiers.
The Web has proven that the concept of hyperlinking is tremendously powerful, but EPUB
Publications have been denied much of the benefit that hyperlinking makes possible
2 © ISO/IEC 2020 – All rights reserved

because of the lack of a standardized scheme to link into them. Although proprietary
schemes have been developed and implemented for individual Reading Systems, without
a commonly-understood syntax there has been no way to achieve cross-platform
interoperability. The functionality that can see significant benefit from breaking down this
barrier, however, is varied: from reading location maintenance to annotation attachment to
navigation, the ability to point into any Publication opens a whole new dimension not
previously available to developers and Authors.
This specification attempts to rectify this situation by defining an arbitrary structural
reference that can uniquely identify any location, or simple range of locations, in an EPUB
Publication: the EPUB CFI. The following considerations have strongly influenced the
design and scope of this scheme:
The mechanism used to reference content should be interoperable: references to a
reading position created by one Reading System should be usable by another.
Document references to EPUB content should be enabled in the same way that
existing hyperlinks enable references throughout the Web.
Each location in an EPUB file should be able to be identified without the need to
modify the document.
All fragment identifiers that reference the same logical location should be equal
when compared.
Comparison operations, including tests for sorting and comparison, should be able
to be performed without accessing the referenced files.
Simple manipulations should be possible without access to the original files (e.g.,
given a reference deep in a file, it should be possible to generate a reference to the
start of the file).
Identifier resolution should be reasonably efficient (e.g., processing of the first
chapter is not necessary to resolve a fragment identifier that points to the last
chapter).
References should be able to recover their target locations through parser variations
and document revisions.
Expression of simple, contiguous ranges should be supported.
An extensible mechanism to accommodate future reference recovery heuristics
should be provided.
In the case of both Standard EPUB CFIs and Intra-Publication EPUB CFI, this
specification conforms with the guidelines expressed by W3C in Section 6. Best Practices
for Fragid Structures [FragIDBestPractices].
In other words, both standard CFI URIs (e.g., "book.epub#epubcfi(…)", referred media type
"application/epub+zip") and intra-publication CFI URIs (e.g., "package.opf#epubcfi(…)",
referred media type "application/oebps-package+xml") make use of a fragment identifier
syntax that does not overlap with existing schemes in the context of the aforementioned
media types' suffix registrations (i.e., "-xml" and "-zip").
› 1.2 Terminology
© ISO/IEC 2020 – All rights reserved 3

Please refer to [EPUB 3.1] for definitions of EPUB-specific terminology used in this
document.
Standard EPUB CFI
A publication-level EPUB CFI links into an EPUB Publication. The path
preceding the EPUB CFI references the location of the EPUB Publication.
Intra-Publication EPUB CFI
An intra-publication EPUB CFI allows one Content Document to reference
another within the same Rendition of an EPUB Publication. The path preceding
the EPUB CFI references the current Rendition's Package Document.
Refer to Intra-Publication CFIs for more information.
› 1.3 Typographic Conventions
The following typographic conventions are used in this specification:
markup
All markup (elements, attributes, properties), code (JavaScript, pseudo-code),
machine-readable values (string, characters, media types) and file names are in red
monospace font.
markup link
Links to markup and code definitions are in underlined red monospace font.
http://www.idpf.org/
URIs are in navy blue monospace font.
hyperlink
Hyperlinks are underlined and blue.
[reference]
Normative and informative references are enclosed in square brackets.
Term
Terms defined in the Terminology are in capital case.
Term Link
Links to term definitions have a dotted blue underline.
Normative element, attribute and property definitions are in blue boxes.
Informative markup examples are in light gray boxes.
4 © ISO/IEC 2020 – All rights reserved

NOTE
Informative notes are in green boxes with a "Note" header.
CAUTION
Informative cautionary notes are in red boxes with a "Caution" header.
› 1.4 Conformance Statements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT,
RECOMMENDED, MAY, and OPTIONAL in this document are to be interpreted as described in
[RFC2119].
All sections and appendixes of this specification are normative except where identified by
the informative status label "This section is informative". The application of informative
status to sections and appendixes applies to all child content and subsections they
contain.
All examples in this specification are informative.
› 2 EPUB CFI Definition
› 2.1 Introduction
This section is informative
A fragment identifier is the part of an IRI [RFC3987] that defines a location within a
resource. Syntactically, it is the segment attached to the end of the resource IRI starting
with a hash (#). For HTML documents, IDs and named anchors are used as fragment
identifiers, while for XML documents the Shorthand XPointer [XPTRSH] notation is used
to refer to a given ID.
A Canonical Fragment Identifier (CFI) is a similar construct to these, but expresses a
location within an EPUB Publication. For example:
book.epub#epubcfi(/6/4[chap01ref]!/4[body01]/10[para05]/3:10)
The function-like string immediately following the hash (epubcfi(…)) indicates that this
fragment identifier conforms to the scheme defined by this specification, and the value
contained in the parentheses is the syntax used to reference the location within the
specified EPUB Publication (book.epub). Using the processing rules defined in Path
Resolution, any Reading System can parse this syntax, open the corresponding Content
Document in the EPUB Publication and load the specified location for the user.
A complete definition of the EPUB CFI syntax is provided in the next section.
© ISO/IEC 2020 – All rights reserved 5

NOTE
epub has been prepended to the name of the scheme, as a more generic CFI-like
scheme might be defined in the future for all XML+ZIP-based file formats.
› 2.2 Syntax
(EBNF productions ISO/IEC 14977)
All terminal symbols are in the Unicode Block 'Basic Latin' (U+0000 to U+007F).
fragment = "epubcfi(" , ( path , [ range ] ) , ")" ;
path = step , local_path ;
range = "," , local_path , "," , local_path ;
local_path = { step } , ( redirected_path | [ offset ] );
redirected_path = "!" , ( offset | path );
step = "/" , integer , [ "[" , assertion , "]" ] ;
offset = ( ( ":" , integer ) | ( "@" , number , ":" , number ) | ( "~" , number
, [ "@" , number , ":" , number ] ) ) , [ "[" , assertion , "]" ] ;
number = ( digit-non-zero , { digit } , [ "." , { digit } , digit-non-zero ] ) | (
zero , [ "." , { digit } , digit-non-zero ] ) ;
integer = zero | ( digit-non-zero , { digit } ) ;
assertion = ( ( value , [ "," , value ] ) | ( "," , value ) | ( parameter ) ) {
parameter } ;
parameter = ";" , value-no-space , "=" , csv ;
csv = value , { "," , value } ;
value = string-escaped-special-chars ;
value-no-space = value - ( [ value ] , space , [ value ] ) ;
special-chars = circumflex | square-brackets | parentheses | comma |
semicolon | equal ;
escaped- = ( circumflex , circumflex ) | ( circumflex , square-brackets ) | (
special-chars circumflex , parentheses ) | ( circumflex , comma ) | (
circumflex , semicolon ) | ( circumflex , equal ) ;
character- = ( character - special-chars ) | escaped-special-chars ;
escaped-
special
string-escaped- = character-escaped-special , { character-escaped-special } ;
special-chars
digit = zero | digit-non-zero ;
6 © ISO/IEC 2020 – All rights reserved

digit-non-zero = "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
zero = "0" ;
space = " " ;
circumflex = "^" ;
square- = "[" | "]" ;
brackets
parentheses = "(" | ")" ;
comma = "," ;
semicolon = ";" ;
equal = "=" ;
character = ? Unicode Characters ? ;
› Unicode Characters
The definition of allowed Unicode characters is the same as [XML]. This excludes the
surrogate blocks, FFFE, and FFFF:

#x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]
Document authors are encouraged to avoid "compatibility characters", as defined in
section 2.3 of [Unicode]. The characters defined in the following ranges are also
discouraged. They are either control characters or permanently undefined Unicode
characters:
[#x7F-#x84], [#x86-#x9F], [#xFDD0-#xFDEF],
[#x1FFFE-#x1FFFF], [#x2FFFE-#x2FFFF], [#x3FFFE-#x3FFFF],
[#x4FFFE-#x4FFFF], [#x5FFFE-#x5FFFF], [#x6FFFE-#x6FFFF],
[#x7FFFE-#x7FFFF], [#x8FFFE-#x8FFFF], [#x9FFFE-#x9FFFF],
[#xAFFFE-#xAFFFF], [#xBFFFE-#xBFFFF], [#xCFFFE-#xCFFFF],
[#xDFFFE-#xDFFFF], [#xEFFFE-#xEFFFF], [#xFFFFE-#xFFFFF],
[#x10FFFE-#x10FFFF].
A Canonical Fragment Identifier (CFI) consists of an initial sequence epubcfi that
identifies this particular reference method, and a parenthesized path or range. A path is
built up as a sequence of structural steps to reference a location. A range is a path
followed by two local (or relative) paths that identify the start and end of the range.
Steps are denoted by the forward slash character (/), and are used to traverse XML
content. The last step in a CFI path represents a location within a document, either
structural (XML element), textual (character data), or aural-visual (image, audio, or video
media). Such terminating steps MAY be complemented by an OPTIONAL "offset", which
denotes a particular character position, temporal or spatial fragment.
Substrings in brackets are extensible assertions that improve the robustness of traversing
paths and migrating them from one revision of the document to another. These assertions
© ISO/IEC 2020 – All rights reserved 7

preserve additional information about traversed elements of the document, which makes it
possible to recover intended location even after some modifications are made to the
EPUB Publication.
Although the value definition in the syntax above allows any a sequence of characters, a
circumflex (^) MUST be used to escape the following characters to ensure their presence
does not interfere with parsing:
brackets ([,])
circumflex (^)
comma (,)
parentheses ((,))
semicolon (;)
Example of an EPUB CFI that points to a location after the text "2[1]".
epubcfi(/6/14[chap05ref]!/4[body01]/10/2/1:3[2^[1^]])
The following rules apply to the use of numbers and integers within the path or range:
leading zeros are not allowed for numbers or integers (to ensure uniqueness);
trailing zeros are not allowed in the fractional part of a number;
zero MUST be represented as the integer 0;
numbers in the range 1 > N > 0 MUST have a leading 0.;
integral numbers MUST be represented as integers.
› 2.3 Character Escaping
As described in Syntax, the EPUB CFI grammar contains characters that have a special
purpose as delimiters within a fragment identifier expression. These characters MUST be
escaped using the circumflex '^' character when not intended for use as delimiters, so that
they can appear within the EPUB CFI data without being mistaken for delimiters.
Depending on the usage context of such EPUB CFI, further character escaping MAY be
necessary in order to ensure that all potentially-conflicting text tokens are encoded
correctly.
IRI and URI references:
The EPUB CFI (fragment identifier) scheme is designed to be used within URI
and IRI references. The [RFC3986] specification defines a number of
"reserved" characters that have a specific purpose as delimiters, and which
MAY need to be escaped in cases when they would otherwise conflict with the
syntactical structure of the URI/IRI reference. The character used for escaping
is the percent sign '%', and escapable characters get percent-encoded. For
example, the percent character itself becomes "%25" when it gets escaped
8 © ISO/IEC 2020 – All rights reserved

(note the difference with EPUB CFI's circumflex '^', which gets escaped using
a double character '^^').
Unlike IRI references, URI references require unicode characters to be ASCII-
encoded. Although the EPUB specification itself is based on IRIs (i.e. authors
and production tools are expected to use IRIs), some systems or APIs might
only support URIs. As a result, implementors MAY still need too handle the
conversion of IRI to URI references, as defined in [RFC3987]. Disallowed
characters are escaped as follows:
Each disallowed character is converted to UTF-8 [RFC2279] as one or
more bytes. The disallowed characters in URI references include all non-
ASCII characters, plus the excluded characters listed in Section 2.4 of
[RFC2396], except for the number sign '#' and percent sign '%' and the
square bracket characters re-allowed in [RFC2732].
The resulting bytes are escaped with the URI escaping mechanism (that
is, converted to '%HH', where HH is the hexadecimal notation of the byte
value).
The original character is replaced by the resulting character sequence.
(X)HTML context:
IRI references are designed to be used in the various types of documents that EPUB
Publications comprise. XML and (X)HTML represent yet another insertion context
that requires specific character escaping rules. For example, double quote
characters or angle brackets conflict with significant delimiters in the markup syntax,
and MUST therefore be escaped using the &xxx; special sequence (character
reference).
When multiple layers of character escaping are applied to escape or unescape an EPUB
CFI, they MUST be applied in reverse order to revert back to the original form. For example,
[ EPUB-CFI -> IRI -> (X)HTML ] becomes [ (X)HTML -> IRI -> EPUB-CFI ]
The following example shows an EPUB CFI in its "raw" form (only with '^' circumflex
escaping). Note the assertion text at the end of it, with escaped square brackets as well as the
escaped circumflex character itself (the unescaped text is 'Ф-"spa ce"-99%-aa[bb]^'):
epubcfi(/6/4!/4/10/2/1:3[Ф-"spa ce"-99%-aa^[bb^]^^])
When taking part in an IRI, the space character within the assertion might become percent-
escaped ('%20'), and the percent character itself MUST be escaped ('%25'). Note that the
square brackets '[' ']' and semicolumn ':' are "reserved" characters (as per the URI
specification) but because they serve no purpose as delimiters when the IRI processor
extracts the fragment identifier, they do not need to be escaped (i.e. the fragment component
of the IRI can non-ambiguously be parsed by processing all the text after the '#' character).
The circumflex '^' also falls within the category of "unwise" (or "unsafe") characters, but the
EPUB fragment identifier scheme does not require escaping them. Here is the IRI-escaped
EPUB CFI:
#epubcfi(/6/4!/4/10/2/1:3[Ф-"spa%20ce"-99%25-aa^[bb^]^^])
When the IRI appears within an XML attribute, the double quote character (quotation mark) is
significant as a delimiter of the attribute value, so it becomes escaped with '"'. Note that
the Cyrillic "EF" character ('Ф') is directly supported in EPUB XML documents (which use the
© ISO/IEC 2020 – All rights reserved 9

UTF-8 encoding to represent the unicode character repertoire), so it doesn't need to be
encoded:
#epubcfi(/6/4!/4/10/2/1:3[Ф-"spa%20ce"-99%25-aa^[bb^]^^])
If the IRI need to be converted to URI, the non-ASCII Cyrillic "EF" character ('Ф') would get
percent-escaped with 2 bytes ('0xd0 0xa4', in hexadecimal). This would result in the
following URI:
#epubcfi(/6/4!/4/10/2/1:3[%d0%a4-%22spa%20ce%22-99%25-aa^[bb^]^^])
URI encoding / decoding APIs usually "aggressively" percent-encode characters, as
demonstrated in the following example. Note how the circumflexes '^' (%5E), square brackets
'[' (%5B) ']' (%5D) and double-quotes '"' (%22) are also percent-encoded (due to their
"unsafe" / "unwise" nature within URIs) :
#epubcfi(/6/4!/4/10/2/1:3%5B%D0%A4-%22spa%20ce%22-99%25-
aa%5E%5Bbb%5E%5D%5E%5E%5D)
› 3 EPUB CFI Processing
› 3.1 Path Resolution
The process of resolving an EPUB CFI to a location within an EPUB Publication begins
with the root package element of the Package Document. Each step in the CFI is then
processed one by one, left to right, applying the rules defined in the following subsections.
NOTE
The EPUB CFI examples in the following subsections are based on the sample
documents in Examples.
› 3.1.1 Step Reference to Child Element or Character Data (/)
A step with a slash (/) followed by a positive integer refers to eith
...