Resolving Lark LALR(1) parsing failures: grammar ambiguity, whitespace/lexer conflicts, and a simple scanner alternative

Why Lark LALR(1) parsing of patch-like annotations fails: grammar ambiguity and whitespace tokenization, plus a robust scanner-based alternative avoids errors.

Parsing patch-like annotations embedded in source files looks straightforward until a production grammar meets real-world constraints. A common pattern is to surround “find” text and its replacement with clearly delimited directives, then scan the file to apply patches. When this file format is fed into a parser generator, subtle ambiguities and lexer rules can collide, producing confusing errors—especially when switching from Earley to LALR(1). This guide unpacks a concrete case, explains why the LALR(1) configuration fails, and shows a pragmatic alternative better suited to the task.

Problem setup

The input format follows a predictable sequence of directives and content. Engineers check in text blocks framed by markers, with optional space or comment-like text around them. A simplified example:

//find_start asdf
a
//find_end

//replace_start
A
//replace_end

//find_start
b
//find_end
this should be ignored
//replace_start
B
//replace_end

//find_start
c
//find_end

//replace_start
C
C
//replace_end

The following Lark grammar parses this structure accurately with the Earley parser. With the LALR(1) parser, however, it crashes at parse time with an unexpected token error.

Grammar demonstrating the issue

Here is the grammar with equivalent logic and renamed symbols. It preserves the original behavior while making identifiers distinct:

root: stanza+

stanza: skipzone? seek_chunk skipzone? swap_chunk skipzone?

seek_chunk: "//find_start" [note_text] payload "//find_end"
swap_chunk: "//replace_start" payload "//replace_end"

_EOL: /\n/+
ROW.-10: /. +/
ANNOT: /\s.+/
payload: (ROW _EOL)+
skipzone: (ROW _EOL)+
note_text: (ANNOT _EOL)

%import common.NEWLINE
%import common.WS
%ignore WS
%ignore NEWLINE

Attempting to parse the sample with LALR(1) produces this error (with names corresponding to the original report):

lark.exceptions.UnexpectedToken: Unexpected token Token('LINE', 'a') at line 3, column 1.
Expected one of: 
        * _NL
Previous tokens: [Token('LINE', 'asdf')]

Why this fails with LALR(1)

There are two distinct issues at play. First, the grammar is ambiguous: the skip zone can be taken as the trailing part of one block or the leading part of the next. Earley can cope with ambiguity; LALR(1) cannot. That alone would typically show up as a parser-construction conflict, but in this case the failure shows at parse time instead.

The immediate cause of the runtime error is whitespace handling. The definitions of the end-of-line and comment-like tokens use regular expressions that include newline and whitespace, while the grammar also declares %ignore WS and %ignore NEWLINE. If the tokenizer is instructed to ignore WS and NEWLINE globally, it cannot simultaneously recognize tokens that rely on those characters, such as the custom end-of-line and comment tokens. The result is that lines collapse into a stream of generic text tokens, exactly as the error indicates when it reports two successive line tokens with the expected end-of-line never materializing. Notably, this contradiction would appear to affect Earley as well; it’s unclear why the same input survives there without error.

A simpler, robust approach

For this use case, a full parser is unnecessary. The structure is linear and directive-driven, so a straightforward scan is both clearer and less error-prone. The following logic walks the file once, echoing or discarding lines according to the markers and producing the intended patched output. The behavior mirrors the described process without introducing grammar ambiguities or whitespace conflicts.

repeat_forever:
  read_and_forward until line_begins_with("//find_start")

  read_and_discard until line_begins_with("//find_end")

  take_one_line_and_drop_it
  ensure_next_line_begins_with("//replace_start")

  read_and_forward until line_begins_with("//replace_end")

end_repeat

Handle end-of-file and error conditions alongside this loop.

Why this matters

Directive-oriented patch formats are better treated as streams than as languages. Trying to coerce them into LALR(1) grammars exposes two kinds of fragility: ambiguity at the block boundaries and lexer-token conflicts caused by global whitespace ignores. A scanner-style solution is easier to maintain, matches the operational model of the patcher, and avoids the parser’s sensitivity to tokenization policy. Where additional complexity is required—such as parameterization and options embedded after //find_start—it can be layered on top of the simple FSM by parsing only those short snippets with a dedicated grammar while the outer file traversal remains a state machine.

Closing advice

If you must keep a grammar, make sure whitespace policy and token definitions do not work at cross purposes, and be mindful that adjacent optional regions often create ambiguity around block boundaries. If you are free to choose the tooling, prefer a linear scan for this directive-driven patch format and reserve parser machinery for the small, self-contained fragments that genuinely need it.