In Python, r’^$’ is a regular expression that matches an empty line
This looks like a regular expression (regex) commonly used in Django URL configurations.
The ‘r’ in front tells Python the expression is a raw string. In a raw string, escape sequences are not parsed. For example, ‘\n’ is a single newline character. But, r’\n’ would be two characters: a backslash and an ‘n’.
Raw strings are handy in regex, in which the backslash is used often for its own purposes. Using ‘[\\w]’ versus r'[\w]’ results in easier to read expressions. This is more salient when you’re escaping backslashes within your regular expression: ‘[\\\\]?’ (shudders).
The ‘^$’ is the actual regular expression, and represents an empty line. The ‘^’ indicates the start of a line, and the ‘$’ indicated the end of a line. Having nothing between the two therefore matches an empty line.
In Django this expression will usually appear at the bottom of your urlpatterns list. It will catch all URLs that weren’t matched by the preceding patterns.
For this particular case, the raw string was unnecessary. Having it here will make it more consistent with urlpatterns that do benefit from being a raw string.
Example: The example code below attempts to explain this, the two entries in the list that match are the completely empty string and the string with just a newline character:
regexp = r'^$'
example_strings = [
'this regular expression should just match empty lines',
'the r prefix just denotes a raw string (to ignore special charcters)',
'the ^ looks for the start of a line',
'the $ looks for the end of a line',
'lets try an empty line to test that theory',
'and a line with just a newline character',
for eg in example_strings:
print('checking: ' + eg)
if re.match(regexp, eg):
print(' it matches')
print(' no match')