I’ve not been able to find anything online to help so hoping someone may have an idea.
What does an underline in an expression mean when using grep?
For example: [_a-zA-Z0-9]
Could someone help to explain the purpose here?
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Answer
The grep command uses a regular expression as it is also described in the manpage of grep:
A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions.
A quick reference of the regular expression syntax can be found here. To test regular expressions with several input strings I recommend regex101.
The pattern [_a-zA-Z0-9] means to match a single character in the list. The list is opened with [ and closed with ]. The underscore (_) has no special meaning it is literally the underscore character. The minus character (-) means range, here from a to z (a-z) for example.
In short [_a-zA-Z0-9] means to match a single character wich is _, a character of the alphabet either lower or uppercase or a numerical character.