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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1. Introduction Skip to TOC

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.¶

2. Objectives Skip to TOC

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 Skip to TOC

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" ) ]

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.¶

4. I-Regexp Semantics Skip to TOC

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 Skip to TOC

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.¶

6. Motivation and Background Skip to TOC

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.¶

7. IANA Considerations Skip to TOC

This document has no IANA actions.¶

8. Security Considerations Skip to TOC

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.¶

Acknowledgements Skip to TOC

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 Skip to TOC