Regex replace in Python - a simpler way

Question

Any time I want to replace a piece of text that is part of a larger piece of text  I always have to do something like      P lt start gt some pattern   P lt replace gt foo   P lt end gt end     And then concatenate the start group with the new data for replace and then the end group   Is there a better method for this

User · Answer

I believe that the best idea is just to capture in a group whatever you want to replace, and then replace it by using the start and end properties of the captured group.

regards

Adrián

#the pattern will contain the expression we want to replace as the first group
pat = "word1\s(.*)\sword2"   
test = "word1 will never be a word2"
repl = "replace"

import re
m = re.search(pat,test)

if m and m.groups() > 0:
    line = test[:m.start(1)] + repl + test[m.end(1):]
    print line
else:
    print "the pattern didn't capture any text"

This will print: 'word1 will never be a word2'

The group to be replaced could be located in any position of the string.

User · Answer

gt  gt  gt  import re  gt  gt  gt  s    start foo end   gt  gt  gt  s   re sub  foo    replaced   s   gt  gt  gt  s  start replaced end   gt  gt  gt  s   re sub     lt                lambda m   can use a callable for the  s text too    m group 1   s   gt  gt  gt  s  start can use a callable for the replaced text too end   gt  gt  gt  help re sub  Help on function sub in module re   sub pattern  repl  string  count 0      Return the string obtained by replacing the leftmost     non-overlapping occurrences of the pattern in string by the     replacement repl   repl can be either a string or a callable      if a callable  it s passed the match object and must return     a replacement string to be used

User · Answer

The short version is that you cannot use variable-width patterns in lookbehinds using Python s re module   There is no way to change this    gt  gt  gt  import re  gt  gt  gt  re sub     lt  foo bar   baz     quux    foobarbaz    fooquuxbaz   gt  gt  gt  re sub     lt  fo  bar   baz     quux    foobarbaz    Traceback  most recent call last     File   lt pyshell 2 gt    line 1  in  lt module gt      re sub     lt  fo  bar   baz     quux   string    File  C  Development Python25 lib re py   line 150  in sub     return  compile pattern  0  sub repl  string  count    File  C  Development Python25 lib re py   line 241  in  compile     raise error  v   invalid expression error  look-behind requires fixed-width pattern   This means that you ll need to work around it  the simplest solution being very similar to what you re doing now    gt  gt  gt  re sub   fo  bar   baz       1quux    foobarbaz    fooquuxbaz   gt  gt  gt   gt  gt  gt    If you need to turn this into a callable function   gt  gt  gt  def replace start  replace  end  replacement  search           return re sub       re escape start          re escape replace            re escape           1    re escape replacement   search    This doesn t have the elegance of the lookbehind solution  but it s still a very clear  straightforward one-liner   And if you look at what an expert has to say on the matter  he s talking about JavaScript  which lacks lookbehinds entirely  but many of the principles are the same   you ll see that his simplest solution looks a lot like this one

User · Answer

Look in the Python re documentation for lookaheads         and lookbehinds    lt       -- I m pretty sure they re what you want  They match strings  but do not  consume  the bits of the strings they match

User · Answer

Look in the Python re documentation for lookaheads         and lookbehinds    lt       -- I m pretty sure they re what you want  They match strings  but do not  consume  the bits of the strings they match

User · Answer

The short version is that you cannot use variable-width patterns in lookbehinds using Python s re module   There is no way to change this    gt  gt  gt  import re  gt  gt  gt  re sub     lt  foo bar   baz     quux    foobarbaz    fooquuxbaz   gt  gt  gt  re sub     lt  fo  bar   baz     quux    foobarbaz    Traceback  most recent call last     File   lt pyshell 2 gt    line 1  in  lt module gt      re sub     lt  fo  bar   baz     quux   string    File  C  Development Python25 lib re py   line 150  in sub     return  compile pattern  0  sub repl  string  count    File  C  Development Python25 lib re py   line 241  in  compile     raise error  v   invalid expression error  look-behind requires fixed-width pattern   This means that you ll need to work around it  the simplest solution being very similar to what you re doing now    gt  gt  gt  re sub   fo  bar   baz       1quux    foobarbaz    fooquuxbaz   gt  gt  gt   gt  gt  gt    If you need to turn this into a callable function   gt  gt  gt  def replace start  replace  end  replacement  search           return re sub       re escape start          re escape replace            re escape           1    re escape replacement   search    This doesn t have the elegance of the lookbehind solution  but it s still a very clear  straightforward one-liner   And if you look at what an expert has to say on the matter  he s talking about JavaScript  which lacks lookbehinds entirely  but many of the principles are the same   you ll see that his simplest solution looks a lot like this one

User · Answer

I believe that the best idea is just to capture in a group whatever you want to replace, and then replace it by using the start and end properties of the captured group.

regards

Adrián

#the pattern will contain the expression we want to replace as the first group
pat = "word1\s(.*)\sword2"   
test = "word1 will never be a word2"
repl = "replace"

import re
m = re.search(pat,test)

if m and m.groups() > 0:
    line = test[:m.start(1)] + repl + test[m.end(1):]
    print line
else:
    print "the pattern didn't capture any text"

This will print: 'word1 will never be a word2'

The group to be replaced could be located in any position of the string.

User · Answer

Look in the Python re documentation for lookaheads         and lookbehinds    lt       -- I m pretty sure they re what you want  They match strings  but do not  consume  the bits of the strings they match

User · Answer

The short version is that you cannot use variable-width patterns in lookbehinds using Python s re module   There is no way to change this    gt  gt  gt  import re  gt  gt  gt  re sub     lt  foo bar   baz     quux    foobarbaz    fooquuxbaz   gt  gt  gt  re sub     lt  fo  bar   baz     quux    foobarbaz    Traceback  most recent call last     File   lt pyshell 2 gt    line 1  in  lt module gt      re sub     lt  fo  bar   baz     quux   string    File  C  Development Python25 lib re py   line 150  in sub     return  compile pattern  0  sub repl  string  count    File  C  Development Python25 lib re py   line 241  in  compile     raise error  v   invalid expression error  look-behind requires fixed-width pattern   This means that you ll need to work around it  the simplest solution being very similar to what you re doing now    gt  gt  gt  re sub   fo  bar   baz       1quux    foobarbaz    fooquuxbaz   gt  gt  gt   gt  gt  gt    If you need to turn this into a callable function   gt  gt  gt  def replace start  replace  end  replacement  search           return re sub       re escape start          re escape replace            re escape           1    re escape replacement   search    This doesn t have the elegance of the lookbehind solution  but it s still a very clear  straightforward one-liner   And if you look at what an expert has to say on the matter  he s talking about JavaScript  which lacks lookbehinds entirely  but many of the principles are the same   you ll see that his simplest solution looks a lot like this one

User · Answer

gt  gt  gt  import re  gt  gt  gt  s    start foo end   gt  gt  gt  s   re sub  foo    replaced   s   gt  gt  gt  s  start replaced end   gt  gt  gt  s   re sub     lt                lambda m   can use a callable for the  s text too    m group 1   s   gt  gt  gt  s  start can use a callable for the replaced text too end   gt  gt  gt  help re sub  Help on function sub in module re   sub pattern  repl  string  count 0      Return the string obtained by replacing the leftmost     non-overlapping occurrences of the pattern in string by the     replacement repl   repl can be either a string or a callable      if a callable  it s passed the match object and must return     a replacement string to be used

User · Answer

The short version is that you cannot use variable-width patterns in lookbehinds using Python s re module   There is no way to change this    gt  gt  gt  import re  gt  gt  gt  re sub     lt  foo bar   baz     quux    foobarbaz    fooquuxbaz   gt  gt  gt  re sub     lt  fo  bar   baz     quux    foobarbaz    Traceback  most recent call last     File   lt pyshell 2 gt    line 1  in  lt module gt      re sub     lt  fo  bar   baz     quux   string    File  C  Development Python25 lib re py   line 150  in sub     return  compile pattern  0  sub repl  string  count    File  C  Development Python25 lib re py   line 241  in  compile     raise error  v   invalid expression error  look-behind requires fixed-width pattern   This means that you ll need to work around it  the simplest solution being very similar to what you re doing now    gt  gt  gt  re sub   fo  bar   baz       1quux    foobarbaz    fooquuxbaz   gt  gt  gt   gt  gt  gt    If you need to turn this into a callable function   gt  gt  gt  def replace start  replace  end  replacement  search           return re sub       re escape start          re escape replace            re escape           1    re escape replacement   search    This doesn t have the elegance of the lookbehind solution  but it s still a very clear  straightforward one-liner   And if you look at what an expert has to say on the matter  he s talking about JavaScript  which lacks lookbehinds entirely  but many of the principles are the same   you ll see that his simplest solution looks a lot like this one

User · Answer

Look in the Python re documentation for lookaheads         and lookbehinds    lt       -- I m pretty sure they re what you want  They match strings  but do not  consume  the bits of the strings they match

User · Answer

gt  gt  gt  import re  gt  gt  gt  s    start foo end   gt  gt  gt  s   re sub  foo    replaced   s   gt  gt  gt  s  start replaced end   gt  gt  gt  s   re sub     lt                lambda m   can use a callable for the  s text too    m group 1   s   gt  gt  gt  s  start can use a callable for the replaced text too end   gt  gt  gt  help re sub  Help on function sub in module re   sub pattern  repl  string  count 0      Return the string obtained by replacing the leftmost     non-overlapping occurrences of the pattern in string by the     replacement repl   repl can be either a string or a callable      if a callable  it s passed the match object and must return     a replacement string to be used

User · Answer

gt  gt  gt  import re  gt  gt  gt  s    start foo end   gt  gt  gt  s   re sub  foo    replaced   s   gt  gt  gt  s  start replaced end   gt  gt  gt  s   re sub     lt                lambda m   can use a callable for the  s text too    m group 1   s   gt  gt  gt  s  start can use a callable for the replaced text too end   gt  gt  gt  help re sub  Help on function sub in module re   sub pattern  repl  string  count 0      Return the string obtained by replacing the leftmost     non-overlapping occurrences of the pattern in string by the     replacement repl   repl can be either a string or a callable      if a callable  it s passed the match object and must return     a replacement string to be used

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