Internet Engineering Task Force (IETF) C. Bormann
Request for Comments: 9485 Universität Bremen TZI
Category: Standards Track T. Bray
ISSN: 2070-1721 Textuality
October 2023
I-Regexp: An Interoperable Regular Expression Format
Abstract
This document specifies I-Regexp, a flavor of regular expression that
is limited in scope with the goal of interoperation across many
different regular expression libraries.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9485.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Terminology
2. Objectives
3. I-Regexp Syntax
3.1. Checking Implementations
4. I-Regexp Semantics
5. Mapping I-Regexp to Regexp Dialects
5.1. Multi-Character Escapes
5.2. XSD Regexps
5.3. ECMAScript Regexps
5.4. PCRE, RE2, and Ruby Regexps
6. Motivation and Background
6.1. Implementing I-Regexp
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
This specification describes an interoperable regular expression
(abbreviated as "regexp") flavor, I-Regexp.
I-Regexp does not provide advanced regular expression features such
as capture groups, lookahead, or backreferences. It supports only a
Boolean matching capability, i.e., testing whether a given regular
expression matches a given piece of text.
I-Regexp supports the entire repertoire of Unicode characters
(Unicode scalar values); both the I-Regexp strings themselves and the
strings they are matched against are sequences of Unicode scalar
values (often represented in UTF-8 encoding form [STD63] for
interchange).
I-Regexp is a subset of XML Schema Definition (XSD) regular
expressions [XSD-2].
This document includes guidance for converting I-Regexps for use with
several well-known regular expression idioms.
The development of I-Regexp was motivated by the work of the JSONPath
Working Group (WG). The WG wanted to include support for the use of
regular expressions in JSONPath filters in its specification
[JSONPATH-BASE], but was unable to find a useful specification for
regular expressions that would be interoperable across the popular
libraries.
1.1. Terminology
This document uses the abbreviation "regexp" for what is usually
called a "regular expression" in programming. The term "I-Regexp" is
used as a noun meaning a character string (sequence of Unicode scalar
values) that conforms to the requirements in this specification; the
plural is "I-Regexps".
This specification uses Unicode terminology; a good entry point is
provided by [UNICODE-GLOSSARY].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The grammatical rules in this document are to be interpreted as ABNF,
as described in [RFC5234] and [RFC7405], where the "characters" of
Section 2.3 of [RFC5234] are Unicode scalar values.
2. Objectives
I-Regexps should handle the vast majority of practical cases where a
matching regexp is needed in a data-model specification or a query-
language expression.
At the time of writing, an editor of this document conducted a survey
of the regexp syntax used in recently published RFCs. All examples
found there should be covered by I-Regexps, both syntactically and
with their intended semantics. The exception is the use of multi-
character escapes, for which workaround guidance is provided in
Section 5.
3. I-Regexp Syntax
An I-Regexp MUST conform to the ABNF specification in Figure 1.
i-regexp = branch *( "|" branch )
branch = *piece
piece = atom [ quantifier ]
quantifier = ( "*" / "+" / "?" ) / range-quantifier
range-quantifier = "{" QuantExact [ "," [ QuantExact ] ] "}"
QuantExact = 1*%x30-39 ; '0'-'9'
atom = NormalChar / charClass / ( "(" i-regexp ")" )
NormalChar = ( %x00-27 / "," / "-" / %x2F-3E ; '/'-'>'
/ %x40-5A ; '@'-'Z'
/ %x5E-7A ; '^'-'z'
/ %x7E-D7FF ; skip surrogate code points
/ %xE000-10FFFF )
charClass = "." / SingleCharEsc / charClassEsc / charClassExpr
SingleCharEsc = "\" ( %x28-2B ; '('-'+'
/ "-" / "." / "?" / %x5B-5E ; '['-'^'
/ %s"n" / %s"r" / %s"t" / %x7B-7D ; '{'-'}'
)
charClassEsc = catEsc / complEsc
charClassExpr = "[" [ "^" ] ( "-" / CCE1 ) *CCE1 [ "-" ] "]"
CCE1 = ( CCchar [ "-" CCchar ] ) / charClassEsc
CCchar = ( %x00-2C / %x2E-5A ; '.'-'Z'
/ %x5E-D7FF ; skip surrogate code points
/ %xE000-10FFFF ) / SingleCharEsc
catEsc = %s"\p{" charProp "}"
complEsc = %s"\P{" charProp "}"
charProp = IsCategory
IsCategory = Letters / Marks / Numbers / Punctuation / Separators /
Symbols / Others
Letters = %s"L" [ ( %s"l" / %s"m" / %s"o" / %s"t" / %s"u" ) ]
Marks = %s"M" [ ( %s"c" / %s"e" / %s"n" ) ]
Numbers = %s"N" [ ( %s"d" / %s"l" / %s"o" ) ]
Punctuation = %s"P" [ ( %x63-66 ; 'c'-'f'
/ %s"i" / %s"o" / %s"s" ) ]
Separators = %s"Z" [ ( %s"l" / %s"p" / %s"s" ) ]
Symbols = %s"S" [ ( %s"c" / %s"k" / %s"m" / %s"o" ) ]
Others = %s"C" [ ( %s"c" / %s"f" / %s"n" / %s"o" ) ]
Figure 1: I-Regexp Syntax in ABNF
As an additional restriction, charClassExpr is not allowed to match
[^], which, according to this grammar, would parse as a positive
character class containing the single character ^.
This is essentially an XSD regexp without:
* character class subtraction,
* multi-character escapes such as \s, \S, and \w, and
* Unicode blocks.
An I-Regexp implementation MUST be a complete implementation of this
limited subset. In particular, full support for the Unicode
functionality defined in this specification is REQUIRED. The
implementation:
* MUST NOT limit itself to 7- or 8-bit character sets such as ASCII,
and
* MUST support the Unicode character property set in character
classes.
3.1. Checking Implementations
A _checking_ I-Regexp implementation is one that checks a supplied
regexp for compliance with this specification and reports any
problems. Checking implementations give their users confidence that
they didn't accidentally insert syntax that is not interoperable, so
checking is RECOMMENDED. Exceptions to this rule may be made for
low-effort implementations that map I-Regexp to another regexp
library by simple steps such as performing the mapping operations
discussed in Section 5. Here, the effort needed to do full checking
might dwarf the rest of the implementation effort. Implementations
SHOULD document whether or not they are checking.
Specifications that employ I-Regexp may want to define in which cases
their implementations can work with a non-checking I-Regexp
implementation and when full checking is needed, possibly in the
process of defining their own implementation classes.
4. I-Regexp Semantics
This syntax is a subset of that of [XSD-2]. Implementations that
interpret I-Regexps MUST yield Boolean results as specified in
[XSD-2]. (See also Section 5.2.)
5. Mapping I-Regexp to Regexp Dialects
The material in this section is not normative; it is provided as
guidance to developers who want to use I-Regexps in the context of
other regular expression dialects.
5.1. Multi-Character Escapes
I-Regexp does not support common multi-character escapes (MCEs) and
character classes built around them. These can usually be replaced
as shown by the examples in Table 1.
+============+===============+
| MCE/class: | Replace with: |
+============+===============+
| \S | [^ \t\n\r] |
+------------+---------------+
| [\S ] | [^\t\n\r] |
+------------+---------------+
| \d | [0-9] |
+------------+---------------+
Table 1: Example
Substitutes for Multi-
Character Escapes
Note that the semantics of \d in XSD regular expressions is that of
\p{Nd}; however, this would include all Unicode characters that are
digits in various writing systems, which is almost certainly not what
is required in IETF publications.
The construct \p{IsBasicLatin} is essentially a reference to legacy
ASCII; it can be replaced by the character class [\u0000-\u007f].
5.2. XSD Regexps
Any I-Regexp is also an XSD regexp [XSD-2], so the mapping is an
identity function.
Note that a few errata for [XSD-2] have been fixed in [XSD-1.1-2];
therefore, it is also included in the Normative References
(Section 9.1). XSD 1.1 is less widely implemented than XSD 1.0, and
implementations of XSD 1.0 are likely to include these bugfixes; for
the intents and purposes of this specification, an implementation of
XSD 1.0 regexps is equivalent to an implementation of XSD 1.1
regexps.
5.3. ECMAScript Regexps
Perform the following steps on an I-Regexp to obtain an ECMAScript
regexp [ECMA-262]:
* For any unescaped dots (.) outside character classes (first
alternative of charClass production), replace the dot with
[^\n\r].
* Envelope the result in ^(?: and )$.
The ECMAScript regexp is to be interpreted as a Unicode pattern ("u"
flag; see Section 21.2.2 "Pattern Semantics" of [ECMA-262]).
Note that where a regexp literal is required, the actual regexp needs
to be enclosed in /.
5.4. PCRE, RE2, and Ruby Regexps
To obtain a valid regexp in Perl Compatible Regular Expressions
(PCRE) [PCRE2], the Go programming language's RE2 regexp library
[RE2], and the Ruby programming language, perform the same steps as
in Section 5.3, except that the last step is:
* Enclose the regexp in \A(?: and )\z.
6. Motivation and Background
While regular expressions originally were intended to describe a
formal language to support a Boolean matching function, they have
been enhanced with parsing functions that support the extraction and
replacement of arbitrary portions of the matched text. With this
accretion of features, parsing-regexp libraries have become more
susceptible to bugs and surprising performance degradations that can
be exploited in denial-of-service attacks by an attacker who controls
the regexp submitted for processing. I-Regexp is designed to offer
interoperability and to be less vulnerable to such attacks, with the
trade-off that its only function is to offer a Boolean response as to
whether a character sequence is matched by a regexp.
6.1. Implementing I-Regexp
XSD regexps are relatively easy to implement or map to widely
implemented parsing-regexp dialects, with these notable exceptions:
* Character class subtraction. This is a very useful feature in
many specifications, but it is unfortunately mostly absent from
parsing-regexp dialects. Thus, it is omitted from I-Regexp.
* Multi-character escapes. \d, \w, \s and their uppercase
complement classes exhibit a large amount of variation between
regexp flavors. Thus, they are omitted from I-Regexp.
* Not all regexp implementations support access to Unicode tables
that enable executing constructs such as \p{Nd}, although the
\p/\P feature in general is now quite widely available. While, in
principle, it is possible to translate these into character-class
matches, this also requires access to those tables. Thus, regexp
libraries in severely constrained environments may not be able to
support I-Regexp conformance.
7. IANA Considerations
This document has no IANA actions.
8. Security Considerations
While technically out of the scope of this specification, Section 10
("Security Considerations") of RFC 3629 [STD63] applies to
implementations. Particular note needs to be taken of the last
paragraph of Section 3 ("UTF-8 definition") of RFC 3629 [STD63]; an
I-Regexp implementation may need to mitigate limitations of the
platform implementation in this regard.
As discussed in Section 6, more complex regexp libraries may contain
exploitable bugs, which can lead to crashes and remote code
execution. There is also the problem that such libraries often have
performance characteristics that are hard to predict, leading to
attacks that overload an implementation by matching against an
expensive attacker-controlled regexp.
I-Regexps have been designed to allow implementation in a way that is
resilient to both threats; this objective needs to be addressed
throughout the implementation effort. Non-checking implementations
(see Section 3.1) are likely to expose security limitations of any
regexp engine they use, which may be less problematic if that engine
has been built with security considerations in mind (e.g., [RE2]).
In any case, a checking implementation is still RECOMMENDED.
Implementations that specifically implement the I-Regexp subset can,
with care, be designed to generally run in linear time and space in
the input and to detect when that would not be the case (see below).
Existing regexp engines should be able to easily handle most
I-Regexps (after the adjustments discussed in Section 5), but may
consume excessive resources for some types of I-Regexps or outright
reject them because they cannot guarantee efficient execution. (Note
that different versions of the same regexp library may be more or
less vulnerable to excessive resource consumption for these cases.)
Specifically, range quantifiers (as in a{2,4}) provide particular
challenges for both existing and I-Regexp focused implementations.
Implementations may therefore limit range quantifiers in
composability (disallowing nested range quantifiers such as
(a{2,4}){2,4}) or range (disallowing very large ranges such as
a{20,200000}), or detect and reject any excessive resource
consumption caused by range quantifiers.
I-Regexp implementations that are used to evaluate regexps from
untrusted sources need to be robust in these cases. Implementers
using existing regexp libraries are encouraged:
* to check their documentation to see if mitigations are
configurable, such as limits in resource consumption, and
* to document their own degree of robustness resulting from
employing such mitigations.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[XSD-1.1-2]
Peterson, D., Ed., Gao, S., Ed., Malhotra, A., Ed.,
Sperberg-McQueen, C. M., Ed., Thompson, H., Ed., and P.
Biron, Ed., "W3C XML Schema Definition Language (XSD) 1.1
Part 2: Datatypes", W3C REC REC-xmlschema11-2-20120405,
W3C REC-xmlschema11-2-20120405, 5 April 2012,
<https://www.w3.org/TR/2012/REC-xmlschema11-2-20120405/>.
[XSD-2] Biron, P., Ed. and A. Malhotra, Ed., "XML Schema Part 2:
Datatypes Second Edition", W3C REC REC-xmlschema-
2-20041028, W3C REC-xmlschema-2-20041028, 28 October 2004,
<https://www.w3.org/TR/2004/REC-xmlschema-2-20041028/>.
9.2. Informative References
[ECMA-262] Ecma International, "ECMAScript 2020 Language
Specification", Standard ECMA-262, 11th Edition, June
2020, <https://www.ecma-international.org/wp-
content/uploads/ECMA-262.pdf>.
[JSONPATH-BASE]
Gössner, S., Ed., Normington, G., Ed., and C. Bormann,
Ed., "JSONPath: Query expressions for JSON", Work in
Progress, Internet-Draft, draft-ietf-jsonpath-base-20, 25
August 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-jsonpath-base-20>.
[PCRE2] "Perl-compatible Regular Expressions (revised API:
PCRE2)", <http://pcre.org/current/doc/html/>.
[RE2] "RE2 is a fast, safe, thread-friendly alternative to
backtracking regular expression engines like those used in
PCRE, Perl, and Python. It is a C++ library.", commit
73031bb, <https://github.com/google/re2>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
[STD63] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
<https://www.rfc-editor.org/info/std63>
[UNICODE-GLOSSARY]
Unicode, Inc., "Glossary of Unicode Terms",
<https://unicode.org/glossary/>.
Acknowledgements
Discussion in the IETF JSONPATH WG about whether to include a regexp
mechanism into the JSONPath query expression specification and
previous discussions about the YANG pattern and Concise Data
Definition Language (CDDL) .regexp features motivated this
specification.
The basic approach for this specification was inspired by "The I-JSON
Message Format" [RFC7493].
Authors' Addresses
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Tim Bray
Textuality
Canada
Email: tbray@textuality.com
