Accessing Groups

Thank you for this tutorial,

I would like to know which method would give as result the two matches, that is:(one,two) and (two,three), I mean recursive matching.

I haven’t read the docs yet, but is there a flag or meta-character in python to alternate between greedy and not greedy matches?

Thank you.

@raulfz Apparently, the built-in re module in Python standard library doesn’t support recursive matching. You can try out a third-party module such as regex, for example.

By default, quantifiers are greedy, but you can make them lazy by appending the question mark (?) meta-character.

Thank you @Bartosz Zaczyński, keep it up with these excellent tutorials!

Best, R.

I don’t understand the use of the 'r' (raw string literal?) in: r"(\w+),(\w+)"

Why is it needed? I understand the regex except for the 'r' prefix.

Thank you

@sheldon Python’s raw strings disable the evaluation of special character sequences denoted with a backslash such as a \", \\, or \n. In regular strings, those are replaced with non-printable characters, such as a newline, or other characters that would lead to a syntactical error:

>>> print("hello\nworld")
hello
world
>>> print('My name\'s Joe.')
My name's Joe.

On the other hand, if you wanted to print such character sequences literally, then you’d need to escape them with another backslash:

>>> print("hello\\nworld")
hello\nworld
>>> print('My name\\\'s Joe.')
My name\'s Joe.

However, that can get cumbersome with regular expressions, which are primarily comprised of special character sequences. Raw strings let you avoid name collisions between special characters, which look the same across standard strings and regular expressions but have a different meaning. For example, the sequence \n in a regular expression is meant to match a newline character rather than insert a literal line break into the pattern.

Thanks for the detailed response Bartosz!

@sheldon – the details of escaping regexes, all the backslashes, and using the raw string are covered in the lesson after this one.

Hi Bartosz,

I am wondering about why the first example applies one backslash to escape while the second applies two?

>>> print("hello\\nworld")
hello\nworld

>>> print('My name\\\'s Joe.')
My name\'s Joe

@Cindy A backslash character affects exactly one character that appears immediately to the right. Therefore, Python will replace two consecutive backslash characters (\\) in a string literal with one backslash in the output:

>>> print("\\")
\

>>> print("hello\\nworld")
hello\nworld

This is necessary because Python treats a single backslash character as the beginning of an escape character sequence such as \n or \t. Without it, you’d get a syntax error, even in raw strings mentioned before:

>>> print("\")
  File "<stdin>", line 1
    print("\")
          ^
SyntaxError: unterminated string literal (detected at line 1)

>>> print(r"\")
  File "<stdin>", line 1
    print(r"\")
          ^
SyntaxError: unterminated string literal (detected at line 1)

In your second example, there are two separate escape character sequences:

>>> print('My name\\\'s Joe.')
My name\'s Joe.

The first one is \\ → \ and the second one is \' → '. Escaping the single quote is required in this case to avoid closing the string literal prematurely. Alternatively, you could enclose the string in double quotes to avoid escaping the single quote:

>>> print("My name\\'s Joe.")
My name\'s Joe.

Hi Cindy,

The first example is escaping a single slash. Normally, “\n” is a reserved marker meant for newline. To actually print a backslash and then an “n”, you have to escape the backslash.

The second example is escaping two things: the first escape is the backlash, the second is the single quote. If I had used double quotes for that example, you wouldn’t need the escaped single-quote.

Notice the difference:

>>> print("My name\\'s Joe.")
My name\'s Joe
>>> print('My name\\\'s Joe.")
My name\s Joe

Same result, but the choice of what kind of quotations to use for the string changes what needs to be escaped.

Evidently @Bartosz and I are replying at the same time using the same examples, how’s that for being in sync :)

@ Bartosz and Christopher, thank you very much for the detailed explanation. It is really helpful!