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Named class needs outer [], e.g. [:digit:](:digit:).

Problematic code:

gzip file[:digit:]*.txt

Correct code:

gzip file[:digit:](:digit:)*.txt

Rationale:

Predefined character groups are supposed to be used inside character ranges. [:digit:] matches one of "digt:" just like [abc] matches one of "abc". [:digit:](:digit:) matches a digit.

Exceptions

When passing an argument to tr which parses these by itself without relying on globbing, you should quote it instead, e.g. tr -d '[:digit:]'

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