在python正则表达式中指定匹配换行的不同方法

Question

I find out there are different ways to match a new line in python regex. For example, all patterns used in the code below can match a new line

str = 'abc\n123'
pattern = '\n'   # print outputs new line 
pattern2 = '\\n' # print outputs \n
pattern3 = '\\\n' # print outputs \ and new line
pattern4 = r'\n'  # print outputs \n
s = re.search(pattern, str).group()
print ('a' + s + 'a')

I have 2 questions about this:

1. pattern is a new line, pattern2 and pattern4 is \\n. Why python regex generates the same pattern for different string?

2. Not sure why pattern3 also generates the same pattern. When passed to re parser, pattern3 stands for \\ + new line, why re parser translates that into just matching new line?

I am using Python 3

Answer 1

The combo \\n indicates a 'newline character' in both Python itself and in re expressions as well ( https://docs.python.org/2.0/ref/strings.html ).

In a regular Python string, \\n gets translated to a newline. The newline code is then fed into the re parser as a literal character.

A double backslash in a Python string gets translated to a single one. Therefore, a string "\\\\n" gets stored internally as "\\n" , and when sent to the re parser, it in turn recognizes this combo \\n as indicating a newline code.

The r notation is a shortcut to prevent having to enter double double backslashes:

backslashes are not handled in any special way in a string literal prefixed with 'r' ( https://docs.python.org/2/library/re.html )