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[markdown] Avoid exponential and quadratic backtracking in tablePattern - #2746

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markdown_table_pattern_backtracking
Oct 29, 2026
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[markdown] Avoid exponential and quadratic backtracking in tablePattern#2746
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markdown_table_pattern_backtracking

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@jonasfj jonasfj commented Oct 29, 2026

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Factor leading horizontal whitespace [ \t]* out of the repeating column
group in tablePattern (pkgs/markdown/lib/src/patterns.dart):

  • Before: r'^[ ]{0,3}\|?([ \t]*:?\-+:?[ \t]*\|[ \t]*)+([ \t]|[ \t]*:?\-+:?[ \t]*)?$'
  • After: r'^[ ]{0,3}\|?[ \t]*(:?\-+:?[ \t]*\|[ \t]*)+(:?\-+:?[ \t]*)?$'

Why this change is necessary

TableSyntax uses tablePattern to test whether the line following a paragraph
line is a GFM table delimiter row (such as | --- | :--- | ---: |). The
previous regex:

r'^[ ]{0,3}\|?([ \t]*:?\-+:?[ \t]*\|[ \t]*)+([ \t]|[ \t]*:?\-+:?[ \t]*)?$'

suffered from both exponential O(2^n) and quadratic O(n^2) catastrophic
backtracking when a table-like line ended with an invalid character:

  1. Exponential O(2^n) backtracking on 'a\n${'-| ' * 30}x':
    Inside the repeating group ([ \t]*:?\-+:?[ \t]*\|[ \t]*)+, [ \t]*
    appeared both at the end of iteration i (after \|) and at the start
    of iteration i + 1 (before :?\-+:?). Whenever a | was followed by at
    least one space or tab before the next column's - or :, that space could
    be matched either by the trailing [ \t]* of iteration i or by the leading
    [ \t]* of iteration i + 1. Across k columns of '-| ' followed by an
    invalid character x at the end of the line, the regex engine explored all
    2^k ways of partitioning those spaces before failing (2^30 > 10^9 steps).
  2. Quadratic O(n^2) backtracking on 'a\n-|${' ' * n}x':
    When a single | was followed by n spaces and an invalid character x, the
    n spaces could be split in n + 1 ways between the trailing [ \t]* of
    ([ \t]*:?\-+:?[ \t]*\|[ \t]*)+ and the leading [ \t]* of
    ([ \t]|[ \t]*:?\-+:?[ \t]*)?$, retrying the tail at each of the n space
    offsets before failing.

How this change works

By consuming [ \t]* once right after ^[ ]{0,3}\|? and keeping [ \t]* only
after \| inside (:?\-+:?[ \t]*\|[ \t]*)+:

  • Every repetition of (:?\-+:?[ \t]*\|[ \t]*)+ and the optional tail
    (:?\-+:?[ \t]*)? must begin strictly with : or - (never [ \t]).
  • All whitespace after | is consumed deterministically by the single
    \|[ \t]*, and the redundant [ \t]| branch in the tail is no longer needed
    because \|[ \t]* already consumes trailing whitespace after the last pipe.
  • The set of matched strings is 100% identical to the previous pattern, and
    matching now runs in strictly O(n) time with zero backtracking ambiguity.

@jonasfj
[markdown] Avoid exponential and quadratic backtracking in tablePattern
c997e17 
Factor leading horizontal whitespace `[ \t]*` out of the repeating column
group in `tablePattern` (`pkgs/markdown/lib/src/patterns.dart`):
- Before: `r'^[ ]{0,3}\|?([ \t]*:?\-+:?[ \t]*\|[ \t]*)+([ \t]|[ \t]*:?\-+:?[ \t]*)?$'`
- After:  `r'^[ ]{0,3}\|?[ \t]*(:?\-+:?[ \t]*\|[ \t]*)+(:?\-+:?[ \t]*)?$'`

----

### Why this change is necessary

`TableSyntax` uses `tablePattern` to test whether the line following a paragraph
line is a GFM table delimiter row (such as `| --- | :--- | ---: |`). The
previous regex:
```dart
r'^[ ]{0,3}\|?([ \t]*:?\-+:?[ \t]*\|[ \t]*)+([ \t]|[ \t]*:?\-+:?[ \t]*)?$'
```
suffered from both **exponential O(2^n)** and **quadratic O(n^2)** catastrophic
backtracking when a table-like line ended with an invalid character:

1. **Exponential O(2^n) backtracking on `'a\n${'-| ' * 30}x'`**:
   Inside the repeating group `([ \t]*:?\-+:?[ \t]*\|[ \t]*)+`, `[ \t]*`
   appeared both at the *end* of iteration `i` (after `\|`) and at the *start*
   of iteration `i + 1` (before `:?\-+:?`). Whenever a `|` was followed by at
   least one space or tab before the next column's `-` or `:`, that space could
   be matched either by the trailing `[ \t]*` of iteration `i` or by the leading
   `[ \t]*` of iteration `i + 1`. Across k columns of `'-| '` followed by an
   invalid character `x` at the end of the line, the regex engine explored all
   2^k ways of partitioning those spaces before failing (`2^30 > 10^9` steps).
2. **Quadratic O(n^2) backtracking on `'a\n-|${' ' * n}x'`**:
   When a single `|` was followed by n spaces and an invalid character `x`, the
   n spaces could be split in `n + 1` ways between the trailing `[ \t]*` of
   `([ \t]*:?\-+:?[ \t]*\|[ \t]*)+` and the leading `[ \t]*` of
   `([ \t]|[ \t]*:?\-+:?[ \t]*)?$`, retrying the tail at each of the n space
   offsets before failing.

### How this change works

By consuming `[ \t]*` once right after `^[ ]{0,3}\|?` and keeping `[ \t]*` only
*after* `\|` inside `(:?\-+:?[ \t]*\|[ \t]*)+`:
- Every repetition of `(:?\-+:?[ \t]*\|[ \t]*)+` and the optional tail
  `(:?\-+:?[ \t]*)?` must begin strictly with `:` or `-` (never `[ \t]`).
- All whitespace after `|` is consumed deterministically by the single
  `\|[ \t]*`, and the redundant `[ \t]|` branch in the tail is no longer needed
  because `\|[ \t]*` already consumes trailing whitespace after the last pipe.
- The set of matched strings is 100% identical to the previous pattern, and
  matching now runs in strictly O(n) time with zero backtracking ambiguity.
@jonasfj
jonasfj requested a review from srawlins as a code owner October 29, 2026 12:58
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Code Review

This pull request updates the tablePattern regular expression in pkgs/markdown/lib/src/patterns.dart to prevent exponential backtracking on table delimiter rows containing many spaces, and documents this fix in the CHANGELOG.md. I have no feedback to provide as there are no review comments and the changes are straightforward.

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@jonasfj
jonasfj merged commit 9da6bb2 into main Oct 29, 2026
24 checks passed
@jonasfj
jonasfj deleted the markdown_table_pattern_backtracking branch October 29, 2026 13:03
copybara-service Bot pushed a commit to dart-lang/sdk that referenced this pull request Nov 3, 2026
@natebosch dart-scoped@luci-project-accounts.iam.gserviceaccount.com
[deps] rev core, ecosystem, http, i18n, shelf, test, tools
c48ebb8 
Revisions updated by `dart tools/rev_sdk_deps.dart`.

core (https://github.com/dart-lang/core/compare/d7bd45c..24e09c4):
  24e09c4c  Sun Nov 1 21:12:19 2026 +0000  dependabot[bot]  Bump the github-actions group with 4 updates (dart-lang/core#1037)
  87ac1b4c  Fri Oct 30 16:02:43 2026 +0100  Michael Thomsen  [os_detect] Deprecate package:os_detect in favor of package:platform (dart-lang/core#1003)
  8debf335  Fri Oct 30 01:57:36 2026 +0530  Sahaja Pallapothula  Clarify Characters equality and Unicode normalization (dart-lang/core#1031)

ecosystem (https://github.com/dart-lang/ecosystem/compare/24b9ba8..f8faabf):
  f8faabf  Sun Nov 1 18:16:20 2026 +0000  dependabot[bot]  Bump the github-actions group with 2 updates (dart-lang/ecosystem#481)
  27c2fc8  Sat Oct 31 12:36:53 2026 +0100  Moritz  Fix publish workflow pins, add tag_prefix to health, and update docs (dart-lang/ecosystem#480)
  05641a9  Sat Oct 31 09:56:42 2026 +0100  Moritz  Restore deprecated publish.yaml workflow and firehose CLI entrypoint (dart-lang/ecosystem#479)
  78abf72  Fri Oct 30 08:24:07 2026 -0700  Kevin Moore  fix(repo_manage): use pub_semver for changelog -wip transitions (dart-lang/ecosystem#475)
  0bfa654  Fri Oct 30 09:33:57 2026 +0100  Moritz  Move publish check to health workflow and use individual package publishing workflows from `setup-dart` (dart-lang/ecosystem#454)
  adf7979  Thu Oct 29 23:46:49 2026 -0700  Kevin Moore  test(firehose): use hasLength, containsPair, and isFalse (dart-lang/ecosystem#477)

http (https://github.com/dart-lang/http/compare/c76dfb3..8c0d66a):
  8c0d66a  Fri Oct 30 09:54:41 2026 -0700  Brian Quinlan  chore(cupertino_http): check ffigen generation (dart-lang/http#2044)
  c8737d1  Fri Oct 30 09:11:46 2026 +0100  Moritz  chore(http): run Firefox tests with -headless instead of xvfb-run (dart-lang/http#2033)
  05d0b18  Fri Oct 30 07:08:19 2026 +0100  Moritz  fix(http2): make StreamException a StreamTransportException and forward ALPN errors to onError (dart-lang/http#2009)

i18n (https://github.com/dart-lang/i18n/compare/4c7b4ad..7475ce6):
  7475ce64  Sat Oct 31 02:49:28 2026 +0100  Moritz  [intl4x] Fix NumberFormat.percent ignoring digits (dart-lang/i18n#1115)

shelf (https://github.com/dart-lang/shelf/compare/62d3e4e..9946de5):
  9946de5  Sun Nov 1 12:27:59 2026 +0000  dependabot[bot]  build(deps): bump the github-actions group with 3 updates (dart-lang/shelf#554)

test (https://github.com/dart-lang/test/compare/6d74720..2a9e017):
  2a9e0175  Sun Nov 1 03:47:24 2026 +0000  dependabot[bot]  Bump the github-actions group with 6 updates (dart-lang/test#2813)
  55cd5cd5  Fri Oct 30 13:43:47 2026 -0700  Nate Bosch  Migrate publishing workflows from firehose to setup-dart (dart-lang/test#2812)
  10dc78c0  Fri Oct 30 11:21:53 2026 -0700  Nate Bosch  [checks] Describe what the stream did in failures (dart-lang/test#2811)
  c11ad06d  Fri Oct 30 08:59:57 2026 -0700  Nate Bosch  [checks] Add CheckFailure.describeNested (dart-lang/test#2793)
  0137b43e  Fri Oct 30 05:40:43 2026 +0530  Shriraj Jadhav  [checks] Use the Iterable.last member for the last expectation (dart-lang/test#2807)
  50c9f454  Thu Oct 29 15:11:39 2026 -0700  Nate Bosch  [checks] Collapse clauses for described conditions (dart-lang/test#2794)

tools (https://github.com/dart-lang/tools/compare/3980dca..1579f8d):
  1579f8d69  Mon Nov 2 02:44:58 2026 +0530  Joshua  fix(coverage): reject package: and dart: URIs outside known roots in Resolver (dart-lang/tools#2764)
  f869fdca4  Sun Nov 1 10:26:34 2026 +0100  dart-ecosystem-bot[bot]  [boring] Update BoringSSL to ed9209cf (dart-lang/tools#2774)
  d10e2b87f  Sun Nov 1 10:26:11 2026 +0100  dart-ecosystem-bot[bot]  [boring] Pin the prebuilt binaries of boring-binaries-20261101-33e9e69722af (dart-lang/tools#2777)
  33e9e6972  Sun Nov 1 05:56:30 2026 +0000  dependabot[bot]  build(deps): bump the github-actions group with 4 updates (dart-lang/tools#2775)
  f84d0d0d2  Sun Nov 1 05:36:12 2026 +0000  dependabot[bot]  build(deps): bump ffigen from 22.0.0 to 23.0.0 in /pkgs/boring (dart-lang/tools#2776)
  14ad5e515  Fri Oct 30 10:48:58 2026 -0700  Kevin Moore  [mime] Harden BoundMultipartStream error routing, headers, and epilogue (dart-lang/tools#2629)
  7a780d48c  Fri Oct 30 12:57:03 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid rescanning failed plain-text runs in InlineParser (dart-lang/tools#2757)
  3671d33a9  Fri Oct 30 12:56:48 2026 +0100  Jonas Finnemann Jensen  [markdown] Index footnote definitions and labels on Document for O(1) lookup (dart-lang/tools#2760)
  6f38e9fda  Fri Oct 30 12:56:09 2026 +0100  Jonas Finnemann Jensen  [markdown] Bound the domain of extended URL autolinks to DNS limits (dart-lang/tools#2759)
  aa0c38544  Fri Oct 30 12:20:40 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic removeRange in _combineAdjacentText (dart-lang/tools#2758)
  f425baf46  Fri Oct 30 11:16:28 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid rescanning failed space runs in LineBreakSyntax (dart-lang/tools#2756)
  c7eafbec3  Fri Oct 30 11:09:45 2026 +0100  Jonas Finnemann Jensen  [markdown] Reject '<' in AutolinkSyntax (dart-lang/tools#2755)
  86ed0f902  Fri Oct 30 11:08:25 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic backtracking in headerPattern (dart-lang/tools#2754)
  8ea07517f  Fri Oct 30 07:45:42 2026 +0100  Moritz  [boring] Regenerate the x509-limbo expected failures in the refresh job (dart-lang/tools#2712)
  40235c365  Thu Oct 29 14:46:17 2026 +0100  Jonas Finnemann Jensen  [markdown] Make HtmlBlockSyntax.parseChildLines iterative (dart-lang/tools#2749)
  0ff03f643  Thu Oct 29 14:41:33 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid re-escaping nested image alt text in ImageSyntax.createNode (dart-lang/tools#2752)
  784654d01  Thu Oct 29 14:37:11 2026 +0100  Jonas Finnemann Jensen  [markdown] Add test cases covering parser and renderer edge cases (dart-lang/tools#2753)
  4f2718ad8  Thu Oct 29 14:36:52 2026 +0100  Jonas Finnemann Jensen  [markdown] Make Document._parseInlineContent iterative (dart-lang/tools#2750)
  cac1421d5  Thu Oct 29 14:17:06 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic duplicate heading ID probing in HtmlRenderer (dart-lang/tools#2748)
  66880bcd3  Thu Oct 29 14:11:27 2026 +0100  Jonas Finnemann Jensen  [markdown] Check footnote label and empty map before scanning footnote keys (dart-lang/tools#2751)
  7f8696523  Thu Oct 29 14:07:06 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic blank-line lookahead in CodeBlockSyntax (dart-lang/tools#2747)
  9da6bb291  Thu Oct 29 14:03:04 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid exponential and quadratic backtracking in tablePattern (dart-lang/tools#2746)

dartdoc is held back at db94437, before dart-lang/dartdoc#4312, because its
mock SDK version changes conflict in the google3 3-way merge with the hotfix
in cl/1005963642.

native is held back at e0ca7c6, before dart-lang/native#3742, because
deprecating `MacOSCodeConfig(targetVersion: ...)` causes a fatal
`deprecated_member_use` diagnostic in `pkg/dartdev/lib/src/native_assets.dart`.

web is held back at 4e60e61, before dart-lang/web#596, because it changes
`package:web` signatures (e.g. `innerHTML=`, `parseFromString`, `Worker()`)
from `JSAny` to `String` in ways that break callers in google3.

Change-Id: I53d2cbde741c45b8fd33bca42742549819dd332b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/572780
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Nate Bosch <[email protected]>
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@szakarias szakarias szakarias approved these changes
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