Calculate the difference and intersection of any two regexes

Re:Calculate the difference and intersection of any two regexes by zeroimpl on Monday September 11, 2023 @10:12PM (#37476283)

Only when the ^ or $ were at the start/end of your string is it simple. Eg:

    (a|b|^)(c|d|^)foo

Rewriting without ^ can require much longer regex.

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Re:Calculate the difference and intersection of any two regexes by wizofaus on Tuesday September 12, 2023 @12:16AM (#37476890)

Isn't that just

    ((a|b)?(c|d)|c|d)?foo

Unless you mean it as a search expression, in which case it's more like

    ((.*a|.*b)(c|d)|c|d)?foo

Which I have to admit was a lot harder to figure out than I thought it would be (and may not even be right!)

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Re:Calculate the difference and intersection of any two regexes by zeroimpl on Tuesday September 12, 2023 @07:43AM (#37479572)

Yeah the latter.

In an engine supporting ^ and $, searching for this

    (a|b|^)(c|d|^)foo

is equivalent to searching for this

    ^((.*a|.*b)(c|d)|c|d)?foo.*$

And in this context you can drop the leading/trailing ^/$ since they are implicit.

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