What Group Does Backreference Refers To When Used With Sub() Operation?

The following code: >>> text = 'imagine a new *world*, a *magic* world' >>> pattern = re.compile(r'\*(.*?)\*') >>> pattern.sub(r'\1<\b>',

Solution 1:

1. sub replaces all matches, not just the first one. From the documentation:

   Return the string obtained by replacing the leftmost non-overlapping occurrences of pattern in string by the replacement repl. [...] The optional argument count is the maximum number of pattern occurrences to be replaced; count must be a non-negative integer. If omitted or zero, all occurrences will be replaced.

2. \b is an escape sequence (for backspace). You should escape it with an other \:

   r'<b>\1<\\b>'

   Used as:

   In [4]: pattern.sub(r'<b>\1<\\b>', text)
   Out[4]: 'imagine a new <b>world<\\b>, a <b>magic<\\b> world'

Escape sequences are interpreted in two different moments:

• By the python compiler, when creating the bytecode, which has to decide which characters to put into the strings.
• By the re engine, when performing the substitution.

To understand why double escaping is required you can try to add one backslash at a time:

In [18]: print(pattern.sub('<b>\\1<\b>', text))
imagine a new <b>world>, a <b>magic> world

In [19]: print(pattern.sub('<b>\\1<\\b>', text))
imagine a new <b>world>, a <b>magic> world

In [20]: print(pattern.sub('<b>\\1<\\\b>', text))
imagine a new <b>world<>, a <b>magic<> world

In [21]: print(pattern.sub('<b>\\1<\\\\b>', text))
imagine a new <b>world<\b>, a <b>magic<\b> world

In [18] the \b is interpreted by the python compiler, so a real backspace character is put in the string (and, as you can see, when replacing it deletes the previous < character)

In [19] the \\ is interpreted as one escaped \ but, afterwards, the re engine sees that you want to replace the some text that contains and escape sequence and reinterprets it, thus yielding the same result as [18].

In [20] the \\ is interpreted as one escaped \ and the final \b as a backspace. The result is that, when replacing, the backspace deletes the \.

In [21] the four \\\\ are interpreted as two escape sequences, which the re engine interprets as a single \ followed by a b (the expected result). Using four \ is equivalent to using raw string literals plus one escaping.

Solution 2:

I went to IRC and people there told me that every time a group is captured with repeated match string, the bacreference is overwritten.