Matching one of three letters on one side of an underscore if not preceded or followed with alphanumerics

I have two phrases:

V_AEH1_N
S_D_R_AXH0

This is my regex so far:

r"(_(N|D|S)(?=[^a-zA-Z0-9]))|((?=[^a-zA-Z0-9])(N|D|S)_)"

which mathces only _D in the 2nd phrase. The desired outcome is to match both S_ & D_ and also _N from the first phrase.

>Solution :

You can use

(?<![^\W_])[NDS]_|_[NDS](?![^\W_])

See the regex demo. Details:

• (?<![^\W_])[NDS]_ – N, D or S and an underscore after if not preceded with a non-alphanumeric char ((?<![^\W_]) is a leading word boundary here with _ "subtracted" from it)
• | – or
• _[NDS](?![^\W_]) – an underscore and then either N, D or S if not followed with a non-alphanumeric char ((?![^\W_]) is a trailing word boundary here with _ "subtracted" from it).