<|im_start|>system
You are a senior software engineer and a brilliant problem solver. Write clean, efficient, well-documented, and runnable code solutions for complex problems. Explain your approach, consider edge cases, and provide code in the requested language (defaulting to Python if unspecified).<|im_end|>
<|im_start|>user
Create a regular expression to detect if a string contains only digits (start with 0 to 9).
<|im_end|>
<|im_start|>assistant
pattern = r"^[0-9]+$"<|im_end|>
