How to use a variable inside a regular expression

Question

I d like to use a variable inside a regex  how can I do this in Python   TEXTO   sys argv 1   if re search r  b    w TEXTO b    w    subject  re IGNORECASE         Successful match else        Match attempt failed

User · Answer

more example  I have configus yml with flows files   pattern     -    d 14     datetime string     -   m d Y H M f    in python code I use  data time real file re findall r   flows flow   pattern   0      latest file

User · Answer

You can use format keyword as well for this Format method will replace    placeholder to the variable which you passed to the format method as an argument    if re search r  b    w        b    w     format TEXTO     subject  re IGNORECASE         Successful match  strong text   else        Match attempt failed

User · Answer

rx   r  b   lt   w  0  b    w   format TEXTO

User · Answer

I find it very convenient to build a regular expression pattern by stringing together multiple smaller patterns   import re  string    begin id1 tag middl id2 tag id3 end  re str1   r    lt    S 5      re str2   r  id d      tag    re pattern   re compile re str1   re str2  match   re pattern findall string  print match    Output      begin    id1      middl    id2

User · Answer

if re search r  b   lt   w  s b    w     TEXTO  subject  re IGNORECASE     This will insert what is in TEXTO into the regex as a string

User · Answer

From python 3 6 on you can also use Literal String Interpolation   f-strings   In your particular case the solution would be   if re search rf  b    w  TEXTO  b    w    subject  re IGNORECASE          do something   EDIT   Since there have been some questions in the comment on how to deal with special characters I d like to extend my answer   raw strings   r     One of the main concepts you have to understand when dealing with special characters in regular expressions is to distinguish between string literals and the regular expression itself  It is very well explained here    In short   Let s say instead of finding a word boundary  b after TEXTO you want to match the string  boundary  The you have to write   TEXTO    Var  subject   r Var boundary   if re search rf  b    w  TEXTO   boundary    w    subject  re IGNORECASE       print  match     This only works because we are using a raw-string  the regex is preceded by  r    otherwise we must write      boundary  in the regex  four backslashes   Additionally  without   r    b  would not converted to a word boundary anymore but to a backspace   re escape   Basically puts a backspace in front of any special character  Hence  if you expect a special character in TEXTO  you need to write   if re search rf  b    w  re escape TEXTO   b    w    subject  re IGNORECASE       print  match     NOTE  For any version    python 3 7                           lt       gt      and   are not escaped  Only special characters with meaning in a regex are still escaped    is not escaped since Python 3 3  s  here   Curly braces   If you want to use quantifiers within the regular expression using f-strings  you have to use double curly braces  Let s say you want to match TEXTO followed by exactly 2 digits   if re search rf  b    w  re escape TEXTO   d  2   b    w    subject  re IGNORECASE       print  match

User · Answer

you can try another usage using format grammer suger   re genre   r     format your variable  regex pattern   re compile re genre

User · Answer

I needed to search for usernames that are similar to each other  and what Ned Batchelder said was incredibly helpful  However  I found I had cleaner output when I used re compile to create my re search term   pattern   re compile r    username                              matches   re findall pattern  lines    Output can be printed using the following   print matches 1     prints one whole matching line  in this case  the first line  print matches 1  3     prints the fourth character group  established with the parentheses in the regex statement  of the first line

User · Answer

I agree with all the above unless    sys argv 1  was something like Chicken d 2 - d 2 An s important s anchor  sys argv 1     Chicken d 2 - d 2 An s important s anchor    you would not want to use re escape  because in that case you would like it to behave like a regex  TEXTO   sys argv 1   if re search r  b   lt   w     TEXTO     b    w    subject  re IGNORECASE         Successful match else        Match attempt failed

User · Answer

You have to build the regex as a string   TEXTO   sys argv 1  my regex   r  b    w     re escape TEXTO    r  b    w    if re search my regex  subject  re IGNORECASE       etc    Note the use of re escape so that if your text has special characters  they won t be interpreted as such

[python] How to use a variable inside a regular expression?

Examples related to python

Examples related to regex

Examples related to python-3.x

Examples related to variables

Examples related to escaping