Regex to Match snake_case Strings

This regex validates strings written in snake_case notation, a naming convention widely used in Python, Ruby, and database column names. Here is the token-by-token breakdown:

^ — Anchors the match at the start of the string.

[a-z] — Matches a single lowercase letter as the first character. Snake case identifiers must start with a letter, not a digit or underscore.

[a-z0-9]* — Matches zero or more lowercase letters or digits for the remainder of the first word segment.

(?:_[a-z0-9]+)* — A non-capturing group that matches zero or more underscore-separated word segments. Each segment consists of a single underscore followed by one or more lowercase letters or digits. This structure prevents consecutive underscores (like my__var), leading underscores, and trailing underscores.

$ — Anchors the match at the end of the string.

No flags are used because case sensitivity is important; snake_case requires all letters to be lowercase.

Snake case is the standard naming convention in Python for variable names, function names, and module names according to PEP 8. It is also commonly used for database column names, JSON API field names in Ruby on Rails, and configuration file keys. Examples include user_name, get_total_count, first_name_last, and api_v2_endpoint.

This pattern enforces clean snake_case formatting without allowing the double-underscore dunder convention used in Python for special methods. For those, a separate pattern would be needed.