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Merged
traviscross merged 12 commits into from
Feb 18, 2026
Merged

Add negative lookahead #2172

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5ee819c
Add negative lookahead to the grammar
ehuss Feb 13, 2026
2a7c350
Add Unicode to character range
ehuss Feb 13, 2026
999f883
Use negative lookahead in the grammar
ehuss Feb 13, 2026
cc7025c
Fix LINE_COMMENT grammar
ehuss Feb 13, 2026
844b827
Fix BLOCK_COMMENT order
ehuss Feb 13, 2026
20f2649
Fix handling of carriage returns in doc comments
ehuss Feb 13, 2026
bff2d5f
Add a new COMMENT grammar rule
ehuss Feb 13, 2026
7c12d35
Fix desugaring of doc comments
ehuss Feb 13, 2026
f57a11f
Add cut to `FLOAT_EXPONENT` and remove reserved alt
traviscross Feb 18, 2026
ae86f19
Fix preposition in `CharacterRange` description
traviscross Feb 18, 2026
fc15897
Fix U+xxxx notation description
traviscross Feb 18, 2026
164f5fa
Add tests for negative lookahead and Unicode
traviscross Feb 18, 2026
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Use negative lookahead in the grammar 
This replaces some suffixes and prose with the new negative lookahead
syntax instead. This should all have the same meaning.
  • Loading branch information
@ehuss @traviscross
ehuss authored and traviscross committed Feb 18, 2026
commit 999f8839e3ae152906c1070344bdb1a802072c81
2 changes: 1 addition & 1 deletion src/identifiers.md
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Expand Up @@ -16,7 +16,7 @@ NON_KEYWORD_IDENTIFIER -> IDENTIFIER_OR_KEYWORD _except a [strict][lex.keywords.
IDENTIFIER -> NON_KEYWORD_IDENTIFIER | RAW_IDENTIFIER

RESERVED_RAW_IDENTIFIER ->
`r#` (`_` | `crate` | `self` | `Self` | `super`) _not immediately followed by XID_Continue_
`r#` (`_` | `crate` | `self` | `Self` | `super`) !XID_Continue
```

<!-- When updating the version, update the UAX links, too. -->
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2 changes: 1 addition & 1 deletion src/input-format.md
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Expand Up @@ -5,7 +5,7 @@ r[input.syntax]
```grammar,lexer
@root CHAR -> [U+0000-U+D7FF U+E000-U+10FFFF] // a Unicode scalar value

@root ASCII -> [U+0000-U+007F]
ASCII -> [U+0000-U+007F]

NUL -> U+0000
```
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30 changes: 13 additions & 17 deletions src/tokens.md
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Expand Up @@ -115,7 +115,7 @@ r[lex.token.literal.suffix.syntax]
```grammar,lexer
SUFFIX -> IDENTIFIER_OR_KEYWORD _except `_`_

SUFFIX_NO_E -> SUFFIX _not beginning with `e` or `E`_
SUFFIX_NO_E -> ![`e` `E`] SUFFIX
```

r[lex.token.literal.suffix.validity]
Expand Down Expand Up @@ -253,8 +253,7 @@ r[lex.token.byte.syntax]
BYTE_LITERAL ->
`b'` ^ ( ASCII_FOR_CHAR | BYTE_ESCAPE ) `'` SUFFIX?

ASCII_FOR_CHAR ->
<any ASCII (i.e. 0x00 to 0x7F) except `'`, `\`, LF, CR, or TAB>
ASCII_FOR_CHAR -> ![`'` `\` LF CR TAB] ASCII

BYTE_ESCAPE ->
`\x` HEX_DIGIT HEX_DIGIT
Expand All @@ -272,8 +271,7 @@ r[lex.token.str-byte.syntax]
BYTE_STRING_LITERAL ->
`b"` ^ ( ASCII_FOR_STRING | BYTE_ESCAPE | STRING_CONTINUE )* `"` SUFFIX?

ASCII_FOR_STRING ->
<any ASCII (i.e 0x00 to 0x7F) except `"`, `\`, or CR>
ASCII_FOR_STRING -> ![`"` `\` CR] ASCII
```

r[lex.token.str-byte.intro]
Expand Down Expand Up @@ -309,8 +307,7 @@ RAW_BYTE_STRING_CONTENT ->
`"` ^ ASCII_FOR_RAW*? `"`
| `#` RAW_BYTE_STRING_CONTENT `#`

ASCII_FOR_RAW ->
<any ASCII (i.e. 0x00 to 0x7F) except CR>
ASCII_FOR_RAW -> !CR ASCII
```

r[lex.token.str-byte-raw.intro]
Expand Down Expand Up @@ -559,7 +556,7 @@ r[lex.token.literal.float.syntax]
FLOAT_LITERAL ->
DEC_LITERAL (`.` DEC_LITERAL)? FLOAT_EXPONENT SUFFIX?
| DEC_LITERAL `.` DEC_LITERAL SUFFIX_NO_E?
| DEC_LITERAL `.` _not immediately followed by `.`, `_` or an XID_Start character_
| DEC_LITERAL `.` !(`.` | `_` | XID_Start)

FLOAT_EXPONENT ->
(`e`|`E`) (`+`|`-`)? `_`* DEC_DIGIT (DEC_DIGIT|`_`)*
Expand Down Expand Up @@ -608,13 +605,12 @@ r[lex.token.literal.reserved.syntax]
RESERVED_NUMBER ->
BIN_LITERAL [`2`-`9`]
| OCT_LITERAL [`8`-`9`]
| ( BIN_LITERAL | OCT_LITERAL | HEX_LITERAL ) `.` _not immediately followed by `.`, `_` or an XID_Start character_
| ( BIN_LITERAL | OCT_LITERAL | HEX_LITERAL ) `.` !(`.` | `_` | XID_Start)
| ( BIN_LITERAL | OCT_LITERAL ) (`e`|`E`)
| `0b` `_`* <end of input or not BIN_DIGIT>
| `0o` `_`* <end of input or not OCT_DIGIT>
| `0x` `_`* <end of input or not HEX_DIGIT>
| `0b` `_`* !BIN_DIGIT
| `0o` `_`* !OCT_DIGIT
| `0x` `_`* !HEX_DIGIT
| DEC_LITERAL ( `.` DEC_LITERAL )? (`e` | `E`) (`+` | `-`)? <end of input or not DEC_DIGIT>
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```

r[lex.token.literal.reserved.intro]
Expand Down Expand Up @@ -657,16 +653,16 @@ r[lex.token.life.syntax]
```grammar,lexer
LIFETIME_TOKEN ->
RAW_LIFETIME
| `'` IDENTIFIER_OR_KEYWORD _not immediately followed by `'`_
| `'` IDENTIFIER_OR_KEYWORD !`'`

LIFETIME_OR_LABEL ->
RAW_LIFETIME
| `'` NON_KEYWORD_IDENTIFIER _not immediately followed by `'`_
| `'` NON_KEYWORD_IDENTIFIER !`'`

RAW_LIFETIME ->
`'r#` IDENTIFIER_OR_KEYWORD _not immediately followed by `'`_
`'r#` ^ IDENTIFIER_OR_KEYWORD !`'`

RESERVED_RAW_LIFETIME -> `'r#` (`_` | `crate` | `self` | `Self` | `super`) _not immediately followed by `'`_
RESERVED_RAW_LIFETIME -> `'r#` (`_` | `crate` | `self` | `Self` | `super`) !(`'` | XID_Continue)
```

r[lex.token.life.intro]
Expand Down