Pattern Matching Exclude Duplicate Characters Is there a regular expression for the following that matches characters in a character set but only once? In other words, once a character is found, remove it from the set. If grep cannot do this, is there a built-in utility which can? Example: Characters to match only once: spine Input: spine spines spin pine seep spins Output: spine spin pine **EDIT:** There are many ways to achieve this output (one example below), but I'm looking for a way to do this without having to customize the command for each pattern I want to match. `grep '[spine]' input_file | grep -v 's.*s' | ... | grep -v 'e.*e'`

With regular expressions in the mathematical sense, it's possible, but the size of the regular expressions grows exponentially relative to the size of the alphabet, so it isn't practical.

There's a simple way with negation and backreferences.


grep '[spine]' | grep -Ev '([spine]).*\1'


The first `grep` selects lines that contain at least one of `einps`; the second `grep` rejects lines that contain more than one of any (e.g. allowing `spinal tap` and `spend` but not `foobar` or `see`).