I-Regexp: An Interoperable Regexp Format
draft-ietf-jsonpath-iregexp-03

Network Working Group                                         C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Standards Track                                 T. Bray
Expires: 8 August 2023                                        Textuality
                                                         4 February 2023

                I-Regexp: An Interoperable Regexp Format
                     draft-ietf-jsonpath-iregexp-03

Abstract

   This document specifies I-Regexp, a flavor of regular expressions
   that is limited in scope with the goal of interoperation across many
   different regular-expression libraries.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-jsonpath-iregexp/.

   Discussion of this document takes place on the JSONPath Working Group
   mailing list (mailto:JSONPath@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/JSONPath/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/JSONPath/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-jsonpath/iregexp.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 8 August 2023.

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Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  I-Regexp Syntax . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Checking Implementations  . . . . . . . . . . . . . . . .   5
   4.  I-Regexp Semantics  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Mapping I-Regexp to Regexp Dialects . . . . . . . . . . . . .   5
     5.1.  Multi-Character Escapes . . . . . . . . . . . . . . . . .   5
     5.2.  XSD Regexps . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  ECMAScript Regexps  . . . . . . . . . . . . . . . . . . .   6
     5.4.  PCRE, RE2, Ruby Regexps . . . . . . . . . . . . . . . . .   7
   6.  Motivation and Background . . . . . . . . . . . . . . . . . .   7
     6.1.  Implementing I-Regexp . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security considerations . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Regexps and Similar Constructs in Recent Published
           RFCs  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This specification describes an interoperable regular expression
   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.

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   I-Regexp supports the entire repertoire of Unicode characters.

   I-Regexp is a subset of XSD regular expressions [XSD-2].

   This document includes guidance for converting I-Regexps for use with
   several well-known regular expression idioms.

1.1.  Terminology

   This document uses the abbreviation "regexp" for what are usually
   called regular expressions in programming.  "I-Regexp" is used as a
   noun meaning a character string which conforms to the requirements in
   this specification; the plural is "I-Regexps".

   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].

2.  Requirements

   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.

   The editors of this document conducted a survey of the regexp syntax
   used in 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.

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   i-regexp = branch *( "|" branch )
   branch = *piece
   piece = atom [ quantifier ]
   quantifier = ( %x2A-2B ; '*'-'+'
    / "?" ) / ( "{" quantity "}" )
   quantity = QuantExact [ "," [ QuantExact ] ]
   QuantExact = 1*%x30-39 ; '0'-'9'

   atom = NormalChar / charClass / ( "(" i-regexp ")" )
   NormalChar = ( %x00-27 / %x2C-2D ; ','-'-'
    / %x2F-3E ; '/'-'>'
    / %x40-5A ; '@'-'Z'
    / %x5E-7A ; '^'-'z'
    / %x7E-10FFFF )
   charClass = "." / SingleCharEsc / charClassEsc / charClassExpr
   SingleCharEsc = "\" ( %x28-2B ; '('-'+'
    / %x2D-2E ; '-'-'.'
    / "?" / %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-10FFFF ) / SingleCharEsc
   catEsc = %s"\p{" charProp "}"
   complEsc = %s"\P{" charProp "}"
   charProp = IsCategory
   IsCategory = Letters / Marks / Numbers / Punctuation / Separators /
       Symbols / Others
   Letters = %s"L" [ ( %x6C-6D ; 'l'-'m'
    / %s"o" / %x74-75 ; 't'-'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" / %x6E-6F ; 'n'-'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 ^.

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   This is essentially XSD regexp without character class subtraction,
   without multi-character escapes such as \s, \S, and \w, and without
   Unicode blocks.

   An I-Regexp implementation MUST be a complete implementation of this
   limited subset.  In particular, full Unicode support 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 non-interoperable syntax, 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 may dwarf the
   rest of the implementation effort.  Implementations SHOULD document
   whether they are checking or not.

   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 which
   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 non-normative, provided as guidance
   to developers who want to use I-Regexps in the context of other
   regular expression dialects.

5.1.  Multi-Character Escapes

   Common multi-character escapes (MCEs), and character classes built
   around them, which are not supported in I-Regexp, can usually be
   replaced as shown for example in Table 1.

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                       +===========+==============+
                       | 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 also is an XSD Regexp [XSD-2], so the mapping is an
   identity function.

   Note that a few errata for [XSD-2] have been fixed in [XSD11-2],
   which is therefore also included as a normative reference.  XSD 1.1
   is less widely implemented than XSD 1.0, and implementations of XSD
   1.0 are likely to include these bugfixes, so 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 dots (.) outside character classes (first alternative of
      charClass production): replace dot by [^\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]).

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   Note that where a regexp literal is required, the actual regexp needs
   to be enclosed in /.

5.4.  PCRE, RE2, Ruby Regexps

   Perform the same steps as in Section 5.3 to obtain a valid regexp in
   PCRE [PCRE2], the Go programming language [RE2], and the Ruby
   programming language, 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 which 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 accesses 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's 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.

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7.  IANA Considerations

   This document makes no requests of IANA.

8.  Security considerations

   As discussed in Section 6, more complex regexp libraries may contain
   exploitable bugs leading to crashes and remote code execution.  There
   is also the problem that such libraries often have hard-to-predict
   performance characteristics, 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]); a
   checking implementation is still RECOMMENDED.

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/rfc/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/rfc/rfc5234>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc8174>.

   [XSD-2]    Malhotra, A., Ed. and P. V. Biron, 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/>.

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   [XSD11-2]  Malhotra, A., Ed., Peterson, D., Ed., Thompson, H., Ed.,
              Sperberg-McQueen, M., Ed., Biron, P. V., Ed., and S. Gao,
              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/>.

9.2.  Informative References

   [ECMA-262] Ecma International, "ECMAScript 2020 Language
              Specification", ECMA Standard ECMA-262, 11th Edition, June
              2020, <https://www.ecma-international.org/wp-
              content/uploads/ECMA-262.pdf>.

   [PCRE2]    "Perl-compatible Regular Expressions (revised API:
              PCRE2)", n.d., <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.", n.d.,
              <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/rfc/rfc7493>.

Appendix A.  Regexps and Similar Constructs in Recent Published RFCs

   This section is to be removed before publishing as an RFC.

   This appendix contains a number of regular expressions that have been
   extracted from some recently published RFCs based on some ad-hoc
   matching.  Multi-line constructions were not included.  With the
   exception of some (often surprisingly dubious) usage of multi-
   character escapes and a reference to the IsBasicLatin Unicode block,
   all regular expressions validate against the ABNF in Figure 1.

   rfc6021.txt  459 (([0-1](\.[1-3]?[0-9]))|(2\.(0|([1-9]\d*))))
   rfc6021.txt  513 \d*(\.\d*){1,127}
   rfc6021.txt  529 \d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d+)?
   rfc6021.txt  631 ([0-9a-fA-F]{2}(:[0-9a-fA-F]{2})*)?
   rfc6021.txt  647 [0-9a-fA-F]{2}(:[0-9a-fA-F]{2}){5}
   rfc6021.txt  933 ((:|[0-9a-fA-F]{0,4}):)([0-9a-fA-F]{0,4}:){0,5}
   rfc6021.txt  938 (([^:]+:){6}(([^:]+:[^:]+)|(.*\..*)))|
   rfc6021.txt 1026 ((:|[0-9a-fA-F]{0,4}):)([0-9a-fA-F]{0,4}:){0,5}
   rfc6021.txt 1031 (([^:]+:){6}(([^:]+:[^:]+)|(.*\..*)))|
   rfc6020.txt 6647 [0-9a-fA-F]*
   rfc6095.txt 2544 \S(.*\S)?

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   rfc6110.txt 1583 [aeiouy]*
   rfc6110.txt 3222 [A-Z][a-z]*
   rfc6536.txt 1583 \*
   rfc6536.txt 1632 [^\*].*
   rfc6643.txt  524 \p{IsBasicLatin}{0,255}
   rfc6728.txt 3480 \S+
   rfc6728.txt 3500 \S(.*\S)?
   rfc6991.txt  477 (([0-1](\.[1-3]?[0-9]))|(2\.(0|([1-9]\d*))))
   rfc6991.txt  525 \d*(\.\d*){1,127}
   rfc6991.txt  541 [a-zA-Z_][a-zA-Z0-9\-_.]*
   rfc6991.txt  542 .|..|[^xX].*|.[^mM].*|..[^lL].*
   rfc6991.txt  571 \d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d+)?
   rfc6991.txt  665 ([0-9a-fA-F]{2}(:[0-9a-fA-F]{2})*)?
   rfc6991.txt  693 [0-9a-fA-F]{2}(:[0-9a-fA-F]{2}){5}
   rfc6991.txt  725 ([0-9a-fA-F]{2}(:[0-9a-fA-F]{2})*)?
   rfc6991.txt  743 [0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-
   rfc6991.txt 1041 ((:|[0-9a-fA-F]{0,4}):)([0-9a-fA-F]{0,4}:){0,5}
   rfc6991.txt 1046 (([^:]+:){6}(([^:]+:[^:]+)|(.*\..*)))|
   rfc6991.txt 1099 [0-9\.]*
   rfc6991.txt 1109 [0-9a-fA-F:\.]*
   rfc6991.txt 1164 ((:|[0-9a-fA-F]{0,4}):)([0-9a-fA-F]{0,4}:){0,5}
   rfc6991.txt 1169 (([^:]+:){6}(([^:]+:[^:]+)|(.*\..*)))|
   rfc7407.txt  933 ([0-9a-fA-F]){2}(:([0-9a-fA-F]){2}){0,254}
   rfc7407.txt 1494 ([0-9a-fA-F]){2}(:([0-9a-fA-F]){2}){4,31}
   rfc7758.txt  703 \d{2}:\d{2}:\d{2}(\.\d+)?
   rfc7758.txt 1358 \d{2}:\d{2}:\d{2}(\.\d+)?
   rfc7895.txt  349 \d{4}-\d{2}-\d{2}
   rfc7950.txt 8323 [0-9a-fA-F]*
   rfc7950.txt 8355 [a-zA-Z_][a-zA-Z0-9\-_.]*
   rfc7950.txt 8356 [xX][mM][lL].*
   rfc8040.txt 4713 \d{4}-\d{2}-\d{2}
   rfc8049.txt 6704 [A-Z]{2}
   rfc8194.txt  629 \*
   rfc8194.txt  637 [0-9]{8}\.[0-9]{6}
   rfc8194.txt  905 Z|[\+\-]\d{2}:\d{2}
   rfc8194.txt  963 (2((2[4-9])|(3[0-9]))\.).*
   rfc8194.txt  974 (([fF]{2}[0-9a-fA-F]{2}):).*
   rfc8299.txt 7986 [A-Z]{2}
   rfc8341.txt 1878 \*
   rfc8341.txt 1927 [^\*].*
   rfc8407.txt 1723 [0-9\.]*
   rfc8407.txt 1749 [a-zA-Z_][a-zA-Z0-9\-_.]*
   rfc8407.txt 1750 .|..|[^xX].*|.[^mM].*|..[^lL].*
   rfc8525.txt  550 \d{4}-\d{2}-\d{2}
   rfc8776.txt  838 /?([a-zA-Z0-9\-_.]+)(/[a-zA-Z0-9\-_.]+)*
   rfc8776.txt  874 ([a-zA-Z0-9\-_.]+:)*
   rfc8819.txt  311 [\S ]+
   rfc8944.txt  596 [0-9a-fA-F]{2}(:[0-9a-fA-F]{2}){7}

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         Figure 2: Example regular expressions extracted from RFCs

Acknowledgements

   This draft has been motivated by the discussion in the IETF JSONPATH
   WG about whether to include a regexp mechanism into the JSONPath
   query expression specification, as well as by previous discussions
   about the YANG pattern and CDDL .regexp features.

   The basic approach for this draft 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

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