Oniguruma (C)

The multi-encoding backtracking engine behind Ruby, jq and several editors. Surveyed for one reason: it is the clearest example of encoding as a first-class parameter rather than an assumption.

Field	Value
Language	C
License	BSD-2-Clause
Repository	kkos/oniguruma
Surveyed revision	f95747b462de672b6f8dbdeb478245ddf061ca53 (v6.9.10-28)
Category	Regex engine (library)
Engine class	Backtracking, with compile-time bounds on pattern features
Guarantee	None; bounded by feature limits and a retry limit

Last reviewed: August 28, 2026.

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Overview

What it solves

Regex over many encodings — UTF-8/16/32, EUC-JP, Shift_JIS, ISO-8859-*, KOI8-R — with syntax dialects selectable per pattern (Ruby, Perl, Java, POSIX). Where other engines assume UTF-8 and treat anything else as the caller's problem, Oniguruma makes the encoding an object the pattern is compiled against.

Design philosophy

Configurability over guarantees. Its bounds are structural caps on the pattern rather than a complexity class:

#define ONIG_MAX_BACKREF_NUM                1000
#define ONIG_MAX_REPEAT_NUM               100000
#define ONIG_MAX_MULTI_BYTE_RANGES_NUM     10000
#define ONIG_MAX_CAPTURE_HISTORY_GROUP        31

[source-verified]

That is a different philosophy from both RE2 (remove the constructs) and PCRE2 (budget the search): Oniguruma caps how much of a construct a pattern may contain, so blowup is bounded at compile time by refusing an over-large pattern.

How it works — what matters here

Encoding is a parameter. OnigEncoding carries the code-point length function, case-folding behaviour and character-class semantics, so \w means something encoding-specific by construction rather than by a Unicode flag.

Syntax is a parameter too. OnigSyntaxType selects the dialect, which is why jq and Ruby can differ while sharing the engine.

Backtracking with a retry limit. The engine is a backtracking matcher; the guard is a configurable retry count, the same shape as PCRE2's match_limit.

The ten dimensions, briefly

Pattern language: dialect-selectable; backreferences, lookaround, named groups, capture history. Engine: backtracking. Prefilter: an optimised start-position search using literal prefixes and a byte map. Corpus access: none. Concurrency: re-entrant with per-match regions. Index: none. Result model: OnigRegion with capture positions, plus capture history as a distinctive extra. Unicode: full, plus non-Unicode encodings. Interactive: none. Measured evidence: none quoted.

Why it is surveyed but not a candidate

Same disqualification as PCRE2 — backtracking, allocation, no time bound — with one additional lesson worth carrying: encoding-parametric matching is expensive and Sparkles does not need it. hue's corpus is source text, which is UTF-8 or is treated as bytes. Recording that Oniguruma pays a real architectural cost for a generality this project can decline is part of scoping the D engine honestly.

The capture-history feature is the one idea with no analogue elsewhere in this catalog: it records every iteration's capture positions, not just the last. It is irrelevant to grep and is noted so it is not re-discovered.

Strengths

• Encoding as a first-class object, not a Unicode boolean.
• Selectable syntax dialects from one engine.
• Compile-time structural caps as a distinct bounding strategy.
• Small, portable, widely embedded.

Weaknesses

• Backtracking: no time bound, retry-limit guarded.
• Allocates during matching (regions, stack).
• Encoding generality costs everywhere, for a property most search tools do not need.

Key design decisions and trade-offs

Decision	Rationale	Trade-off
Encoding as a parameter object	Correct matching in non-Unicode encodings	Indirection on every character operation
Selectable syntax dialects	One engine serves Ruby, Perl and POSIX users	Behaviour depends on a second, non-obvious parameter
Structural caps on the pattern	Bounds blowup at compile time	Caps are arbitrary constants, not derived from a budget
Capture history	Enables iteration-aware submatch analysis	Storage per iteration; useless for line search

Sources

Read at f95747b462de672b6f8dbdeb478245ddf061ca53 [source-verified]:

• src/oniguruma.h — ONIG_MAX_* caps, encoding and syntax objects, OnigRegion