fix: count numbered section headers in validate_sections

[Chessing234]

Summary

• validate_sections now requires Section N-style headers and >= N sections, matching IFEvalG.

Test plan

• Unit tests for intro + numbered headers vs bare splitter
• GPU_TESTS=bypass

[gemini-code-assist]

Code Review

This pull request updates the validate_sections function in open_instruct/if_functions.py to identify numbered section headers using a regular expression, ensuring that headers contain the splitter followed by a number. It also adds unit tests to verify this behavior and updates the changelog. The review feedback points out that the regular expression should be case-insensitive to support headers like SECTION 2 as described in the docstring, and suggests adding a test case to verify case-insensitive matching.

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[gemini-code-assist]

The docstring states that headers can look like Section 1 or SECTION 2. However, without passing re.IGNORECASE to the regex engine, the matching is case-sensitive. If the section_splitter is "Section", it will fail to match SECTION 2. Compiling the pattern with re.IGNORECASE ensures that case-insensitive headers are correctly matched as described in the docstring.

    pattern = r"\\s?" + re.escape(section_splitter) + r"\\s?\\d+\\s?"\n    sections = re.compile(pattern, re.IGNORECASE).split(text)

[gemini-code-assist]

Add a test case to verify that case-insensitive section headers (e.g., SECTION 2 when splitter is Section) are correctly validated, matching the behavior described in the docstring.

Suggested change

	("numbered_with_intro", "Intro\nSection 1\nHello\nSection 2\nWorld", 2, "Section", True),
	("bare_splitter_not_enough", "Intro\nSection\nHello\nSection\nWorld", 2, "Section", False),
	("at_least_n", "Section 1\na\nSection 2\nb\nSection 3\nc", 2, "Section", True),
	("too_few", "Section 1\nonly one", 2, "Section", False),
	("numbered_with_intro", "Intro\\nSection 1\\nHello\\nSection 2\\nWorld", 2, "Section", True),\n ("case_insensitive", "Intro\\nSection 1\\nHello\\nSECTION 2\\nWorld", 2, "Section", True),\n ("bare_splitter_not_enough", "Intro\\nSection\\nHello\\nSection\\nWorld", 2, "Section", False),\n ("at_least_n", "Section 1\\na\\nSection 2\\nb\\nSection 3\\nc", 2, "Section", True),\n ("too_few", "Section 1\\nonly one", 2, "Section", False),