Question

Using Python I need to delete all characters in a multiline string up to the first occurrence of a given pattern. In Perl this can be done using regular expressions with something like:

#remove all chars up to first occurrence of cat or dog or rat
$pattern = 'cat|dog|rat' 
$pagetext =~ s/(.*?)($pattern)/$2/xms; 

What’s the best way to do it in Python?

Answer 1

You want to delete all characters preceding the first occurrence of a pattern; as an example, you give “cat|dog|rat”.

Code that achieves this using re:

re.sub("(?s).*?(cat|dog|rat)", "\\1", input_text, 1)

or, if you’ll be using again this regular expression:

rex= re.compile("(?s).*?(cat|dog|rat)")
result= rex.sub("\\1", input_text, 1)

Note the non-greedy .*?. The initial (?s) allows to match newline characters too, before the word matching.

Examples:

>>> input_text= "I have a dog and a cat"
>>> re.sub(".*?(cat|dog|rat)", "\\1", input_text, 1)
'dog and a cat'

>>> re.sub("(?s).*?(cat|dog|rat)", "\\1", input_text, 1)
'I have no animals!'

>>> input_text= "This is irrational"
>>> re.sub("(?s).*?(cat|dog|rat)", "\\1", input_text, 1)
'rational'

In case you want to do the conversion only for the words cat, dog and rat, you’ll have to change the regex into:

>>> re.sub(r"(?s).*?\b(cat|dog|rat)\b", "\\1", input_text, 1)
'This is irrational'