python - How to apply re.sub() with exceptions

Question

I'm trying to program a vocabulary game.

I am using a regular expression to hide my word, which I have to guess. I'm not comfortable with the syntax used with regular expressions - outwith the simple examples, I get very confused.

Take for example the verb

'to crank (sth) up'

I want to transform that into:

to   _ _ _ _ _   (sth)  _ _ 

The programme will feed from a vocabulary CSV file. My convention is to add (sth) or (smb) for transitive verbs. I don't want to hide those bits between brackets. Likewise, I don't want to hide the to that denotes the infinitive tense.

The transformations I'm applying so far are:

chosen_word = "to crank (sth) up"

# To make the space between words double for better legibility
hidden_word = re.sub("s", "  ", chosen_word)

# To hide the letters of the word 
hidden_word = re.sub("[a-z]", "_ ", hidden_word)

But that results in:

_ _    _ _ _ _ _   ( _ _ _ )  _ _

How can I code a re.sub() method that transforms all alphabetical characters to _ except the patterns to and sth and smb?

Answer 1

You can capture the exclusions and then use a dynamic replacement pattern:

hidden_word = re.sub(r"(to|(s(?:th|b)))|[a-z]", lambda x: x.group(1) or "_ ", hidden_word)

See the Python demo. Regex details:

• (to|(s(?:th|b))) - Group 1: either a whole word to or (sth) or (sb)
• | - or
• [a-z] - a lowercase ASCII letter
• lambda x: x.group(1) or "_ " - the match is either replaced with Group 1 value (if it was matched) or with an underscore plus space otherwise.