Why do I need to add DOTALL to python regular expression to match new line in raw string

Question

Why does one need to add the DOTALL flag for the python regular expression to match characters including the new line character in a raw string. I ask because a raw string is supposed to ignore the escape of special characters such as the new line character. From the docs:

The solution is to use Python’s raw string notation for regular expression patterns; backslashes are not handled in any special way in a string literal prefixed with 'r'. So r"\n" is a two-character string containing '\' and 'n', while "\n" is a one-character string containing a newline.

This is my situation:

string = '\nSubject sentence is:  Appropriate support for families of children diagnosed with hearing impairment\nCausal Verb is :  may have\npredicate sentence is:  a direct impact on the success of early hearing detection and intervention programs in reducing the negative effects of permanent hearing loss'

re.search(r"Subject sentence is:(.*)Causal Verb is :(.*)predicate sentence is:(.*)", string ,re.DOTALL)

results in a match , However , when I remove the DOTALL flag, I get no match.

Answer 1

In regex . means any character except \n

So if you have newlines in your string, then .* will not pass that newline(\n).

But in Python, if you use the re.DOTALL flag(also known as re.S) then it includes the \n(newline) with that dot .

Answer 2

Your source string is not raw, only your pattern string.

maybe try

string = r'\n...\n'
re.search("Subject sentence is:(.*)Causal Verb is :(.*)predicate sentence is:(.*)", string)