Question

I have two regular expressions, one matching for all characters [a-z] and the other excluding the following combination of characters [^spuz(ih)] (the characters s, p, u, z, ih)how would I combine these two so that I could allow all alphanumeric characters except those listed in the second RE?

(re.match(r'^[a-z]*(?![spuz]|ih)[a-z]s$', insert_phrase)

Answer 1

You can’t “combine” them as such, but you can write another regular expression which has the same effect. For this, you can use the (?!) construct. It matches 0 characters only if the regular expression in it is not matched by the following part. So you can use:

'(?![spuz(ih)])[a-z]'

Or, since this wasn’t what you wanted, change it to:

'(?![spuz]|ih)[a-z]'

In the changed question, you seem to want negative lookbehind instead. This turns the pattern into:

'^[a-z]*(?<![a-z][spuz]|ih)s$'

Note the extra [a-z] in the lookbehind part. It is required because lookbehind expressions must be fixed width. This means that a string like ‘ps’ will match the pattern, but you don’t want that. So instead, it’s better to use two separate lookbehinds (both of which have to be be true for the string to match):

'^[a-z]*(?<![spuz])(?<!ih)s$'