regex error - nothing to repeat

Question

I get an error message when I use this expression   re sub r     s w    s  1       1           I checked the regex at RegExr and it returns   as expected   But when I try it in Python I get this error message   raise error  v   invalid expression sre constants error  nothing to repeat   Can someone please explain

User · Answer

regular expression normally uses   and   in theory of language   I encounter the same bug while executing the line code   re split     text    to solve it  it needs to include   before   and    re split      text

User · Answer

That is a Python bug between     and special characters   Instead of  re compile r  w      Try   re compile r  a-zA-Z0-9       It works  however does not make the same regular expression   This bug seems to have been fixed between 2 7 5 and 2 7 6

User · Answer

Beyond the bug that was discovered and fixed  I ll just note that the error message sre constants error  nothing to repeat is a bit confusing  I was trying to use r      as a pattern  and thought it was complaining for some strange reason about the    but the problem is actually that   is a way of saying  repeat zero or one times   So I needed to say r      to match a literal

User · Answer

It seems to be a python bug  that works perfectly in vim   The source of the problem is the   s       bit  Basically   you can t do   s    which make sense   because you are trying to repeat something which can be null    gt  gt  gt  re compile r   s      Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   System Library Frameworks Python framework Versions 2 5 lib python2 5 re py   line 180  in compile     return  compile pattern  flags    File   System Library Frameworks Python framework Versions 2 5 lib python2 5 re py   line 233  in  compile     raise error  v   invalid expression sre constants error  nothing to repeat   However   s  1  should not be null   but we know it only because we know what s in  1  Apparently python doesn t     that s weird

User · Answer

It s not only a Python bug with   actually  it can also happen when you pass a string as a part of your regular expression to be compiled  like    import re input line    string from any input source  processed line   text to be edited with     format input line  target    text to be searched  re search processed line  target    this will cause an error if processed line contained some       for example  like you can find in chemical formulae  or such chains of characters  the solution is to escape but when you do it on the fly  it can happen that you fail to do it properly

[python] regex error - nothing to repeat

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