Regular Expressions

Raw strings are just another way to write string literals.

Regular string literals

As you surely know already, Python gives lots of ways to write string literals:

Using single quotes

 #with single quotes
 my_variable = 'my value"

Using double quotes

 #with double quotes
 my_variable = "my value"

Using triple quotes

#with triple single quotes
my_variable = '''my value'''

#with triple double quotes
my_variable = """my value"""

The problem: backslashes

In regular string literals, as you surely know already, backslashes have special meaning. They indicate an escape character.

 #a string with a line break in it, since \n in a string literal represents a line break
 waste_of_my_time = 'this\nexample'

 #a string with a tab in it, since \t in a string literal represents a line break
 work_of_art = 'this\twebsite'

 #a string with two double quotes in it, since \" is the escape character for a double quote
 genius_advice = "She said, \"Don't worry about it, it's ornithological!\""

 #another string with an apostrophe (same character as single quote), since \' is an escaped single quote
 more_genius_advice = 'She said, "Don't worry about it, it\'s ornithological!"'

 #a string with a single backslash in it, since \\ represents one backslash!
 getting_dizzy = 'C:\\Program Files'

 #lots of escaped backslashes become difficult to read
 caput = 'C:\\Program Files\\^%(x86)\\Internet Explorer\\iexplore.exe'

and so on...

The solution: raw strings

In raw string literals, backslashes have no special meaning as an escape character. This is useful for writing strings that contain a lot of backslashes. Not having to escape each backslash makes the string more readable.

 #notice the r prefixe indicates a raw string literal
 less_caput = r'C:\Program Files\^%(x86)\Internet Explorer\iexplore.exe'

Regular expressions are more easily written with raw string literals

Regular expressions usually contain a lot of backslashes. When using Python's re module, regular expressions are represented as strings. So, like all strings with a lot of backslashes, they are more readable when written in raw literal form.

 #import Python's regular expressions module
 import regex

 #compile a regular expression... note the raw string literal that doesn't have its backslashes escaped
 p = re.compile(r'[Ee](\+|-)?[0-9]+')

In the above example, if we hadn't written regular expressions as a raw string, we would have had to escape any backslashes, which would have made the string even more difficult to read than it already is.

Net net

Net net, whichever way you do it, your goal is to ultimately have a string representing the regular expression as it is properly written in regular expressions syntax.