Add string slicing support

[Copilot]

yq supported array slicing (.[1:3]) but not string slicing — .field[4:] on a string value silently returned the wrong result or panicked.

Changes

• operator_slice.go — Added sliceStringNode (rune-based for Unicode correctness); sliceArrayOperator now routes !!str scalar nodes to it instead of the array path.
• operator_traverse_path.go — Fixed traverseArrayOperator to evaluate the LHS before calling sliceArrayOperator. Previously it passed the raw document context, so .country[4:] would operate on the whole map rather than the country value.
• lexer.go — Extended the implicit-0 insertion (for omitted start index in [:N]) to fire on openCollect ([) in addition to traverseArrayCollect (.[), enabling .field[:N] syntax.
• operator_slice_test.go — Added string slicing scenarios: start-only, end-only, both, negative indices, out-of-bounds clamping, full [:], and Unicode.
• doc/operators/headers/slice-array.md — Updated header to reflect string support.

Example

# sample.yml
country: Australia

yq '.country[4:]'   sample.yml  # ralia
yq '.country[:5]'   sample.yml  # Austr
yq '.country[-5:]'  sample.yml  # ralia
yq '.country[1:-1]' sample.yml  # ustrali

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[mikefarah]

Test says without the second number, but both numbers are present

[mikefarah]

yep this has the second number...should this be .country[5:] ?

[mikefarah]

@copilot fix this ^;

also the build is failing with:

Error: pkg/yqlib/operator_slice.go:19:98: sliceStringNode - result 1 (error) is always nil (unparam)
func sliceStringNode(lhsNode *CandidateNode, firstNumber int, secondNumber int) (*CandidateNode, error) {

[Copilot]

Fixed in 9a9399a:

• Changed "Slicing strings - without the second number" expression from .country[0:5] to .country[5:] (now actually omits the second number, giving alia)
• Changed the first "Slicing strings" test to use both numbers: .country[0:5] → Austr
• Removed the always-nil error return from sliceStringNode to fix the build warning

[Copilot]

Pull request overview

Adds correct string-slicing behavior to yq’s slice operator and fixes traversal so slice operations apply to the evaluated LHS value (e.g. .country[4:] slices the country string, not the root document).

Changes:

• Add rune-based string slicing support and route !!str scalar nodes through it.
• Fix traverseArrayOperator to evaluate/pipeline the LHS before applying slice logic.
• Extend lexer post-processing to support omitted-start string slicing syntax like .field[:N].

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
project-words.txt	Adds new words used by tests/docs to the project dictionary.
pkg/yqlib/operator_traverse_path.go	Ensures slice operations receive the evaluated LHS context.
pkg/yqlib/operator_slice.go	Introduces sliceStringNode and routes string scalars through it.
pkg/yqlib/lexer.go	Enables implicit 0 insertion for [:N] after [ (not just after .[).
pkg/yqlib/operator_slice_test.go	Adds coverage for string slicing (including Unicode and bounds cases).
pkg/yqlib/doc/operators/slice-array.md	Updates generated operator docs to include string slicing examples.
pkg/yqlib/doc/operators/headers/slice-array.md	Updates the slice operator header to reflect string support.

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[Copilot]

Array slicing can still panic when the start index is more negative than the array length (e.g. .[-100:]): relativeFirstNumber is adjusted by len(lhsNode.Content)+firstNumber but is not clamped back to 0, so the loop can index lhsNode.Content with a negative i. Clamp relativeFirstNumber (and similarly relativeSecondNumber) into [0,len] before iterating to avoid runtime panics and match the existing out-of-bounds clamping behavior for other cases.

[mikefarah]

@copilot apply changes based on this feedback

[Copilot]

Fixed in 341e252. Array slice now clamps both relativeFirstNumber and relativeSecondNumber into [0, len(Content)] after adjusting for negative indices, preventing a panic on expressions like .[-100:]. Also added test cases for both directions of out-of-bounds negative indices.