I am currently learning regular expression in Python, and I am confused by what regular expression really matches. For example, '\\' is actually '\', so what if I need to match the '\' in string 'abc\\cde'? re.findall(r'\', '010\\aa') doesn't work at all. Thanks for any help.
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Wiktor Stribiżew
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Jieyu You
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`re.findall(r'\\', '010\\aa')` – cs95 Jul 14 '17 at 21:40
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You can't have a trailing backslash in a raw string: https://stackoverflow.com/questions/647769/why-cant-pythons-raw-string-literals-end-with-a-single-backslash – jonrsharpe Jul 14 '17 at 21:40
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Pythex
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@jonrsharpe Now I understand what's wrong with my code, thank you! – Jieyu You Jul 14 '17 at 22:38
2 Answers
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As others have mentioned, \ is one character you still need to escape in raw strings:
re.findall(r'\\', '010\\aa')
Note that r'\\' is a string of length two, containing two backslashes, not one, unlike '\\'. This is described at the bottom of https://docs.python.org/2.0/ref/strings.html .
aaronm04
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`len(r'\x07')` is 4 characters, so it is not equivalent to `'\a'`. I think the regular expression library is doing its own conversion so that this can match `'\a'`. – aaronm04 Jul 14 '17 at 22:37
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And Python refuses to remove escapes when it parses raw literals, which is dumb. – Jul 15 '17 at 02:43
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You always need to escape the delimiter in strings.
Since languages need to parse an escaped delimiter,
this requires parsing the escape itself.
This is a fact of language source string parsing, even raw strings.
Examples:
' => r'\''
\' => r'\''
\\' => r'\\\''