Python re sub how to substitute all u or U s with you

Question

I am doing some text normalization using python and regular expressions  I would like to substitute all  u or  U s with  you    Here is what I have done so far   import re text    how are u  umberella u  u  U  U  U  u   print re sub     u U  s       W       a-zA-Z       you    text    The output I get is   how are you  you berella you  you  you  you  you  you   As you can see the problem is that  umberella  is changed to  berella   Also I want to keep the character that appears after a  u   For example I want  u   to be changed to  you    Can anyone tell me what I am doing wrong and what is the best way to write the regular expression

User · Answer

This worked for me       import re     text    how are u  umberella u  u  U  U  U  u       rex   re compile r  bu b   re IGNORECASE      print rex sub  you   text     It pre-compiles the regular expression and makes use of re IGNORECASE so that we don t have to worry about case in our regular expression   BTW  I love the funky spelling of umbrella   -

User · Answer

Firstly  why doesn t your solution work  You mix up a lot of concepts  Mostly character class with other ones  In the first character class you use   which stems from alternation  In character classes you don t need the pipe  Just list all characters  and character ranges  you want    Uu    Or simply write u if you use the case-insensitive modifier  If you write a pipe there  the character class will actually match pipes in your subject string   Now in the second character class you use the comma to separate your characters for some odd reason  That does also nothing but include commas into the matchable characters  s and W are probably supposed to be the built-in character classes  Then escape them  Otherwise they will just match literal s and literal W  But then  W already includes everything else you listed there  so a  W alone  without square brackets  would have been enough  And the last part   a-zA-Z  also doesn t work  because it will simply include         and all letters into the character class  The negation syntax only works for entire character classes like   a-zA-Z    What you actually want is to assert that there is no letter in front or after your u  You can use lookarounds for that  The advantage is that they won t be included in the match and thus won t be removed   r    lt   a-zA-Z   uU     a-zA-Z      Note that I used a raw string  Is generally good practice for regular expressions  to avoid problems with escape sequences   These are negative lookarounds that make sure that there is no letter character before or after your u  This is an important difference to asserting that there is a non-letter character around  which is similar to what you did   because the latter approach won t work at the beginning or end of the string   Of course  you can remove the spaces around you from the replacement string   If you don t want to replace u that are next to digits  you can easily include the digits into the character classes   r    lt   a-zA-Z0-9   uU     a-zA-Z0-9      And if for some reason an adjacent underscore would also disqualify your u for replacement  you could include that as well  But then the character class coincides with the built-in  w   r    lt   w  uU     w     Which is  in this case  equivalent to EarlGray s r  b uU  b    As mentioned above you can shorten all of these  by using the case-insensitive modifier  Taking the first expression as an example   re sub r    lt   a-z  u    a-z      you   text  flags re I    or  re sub r    lt   a-z  u    a-z      you   text  flags re IGNORECASE    depending on your preference   I suggest that you do some reading through the tutorial I linked several times in this answer  The explanations are very comprehensive and should give you a good headstart on regular expressions  which you will probably encounter again sooner or later

User · Answer

Use a special character  b  which matches empty string at the beginning or at the end of a word    print re sub r  b uU  b    you   text    spaces are not a reliable solution because there are also plenty of other punctuation marks  so an abstract character  b was invented to indicate a word s beginning or end

User · Answer

it can also be achieved with below code  import re  text    how are u  umberella u  u  U  U  U  u   print  re sub  r  uU      a-z      r  you 1    text     or  print  re sub  r  uU      s            r  you 1    text

User · Answer

Another possible solution I came up with was    re sub r   uU       s     you    text

[python] Python re.sub(): how to substitute all 'u' or 'U's with 'you'

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