Perl regex in Python

Question

I'm having problems with regex conversion from Perl to Python. I have this code:

my $str_old = my $str = '$x start $x bar$xy $$x end $x';

$str =~ s/(\s|^)(\$x)(\s|$)/$1\$Q$3/g;

print $str_old."\n";
print $str."\n";

and it should output this

$x start $x bar$xy $$x end $x
$Q start $Q bar$xy $$x end $Q

But I just can't get it working in Python.

I got into this state, but it remove white spaces> import re str_old = str = '$x start $x bar$xy $$x end $x'; str = re.sub( '(\s|^)(\$x)(\s|$)' , '$Q', str) print (str) print (str_old) — Tomas Bruckner
– Tomas Bruckner, Commented Mar 29, 2014 at 11:41

Jerry · Accepted Answer · 2014-03-29 16:29:20Z

1

Try this:

import re
str_old = str = '$x start $x bar$xy $$x end $x'  # You can remove this semicolon
 # str = re.sub( '(\s|^)(\$x)(\s|$)' , '$Q', str)
 #                                      ^^ You're not placing the spaces back.
str = re.sub( '(\s|^)\$x(\s|$)' , '\\1$Q\\2', str)

print(str)
print(str_old)

That said, you should raw your regex string and replacement string:

str = re.sub(r'(\s|^)\$x(\s|$)' , r'\1$Q\2', str)

And last, avoid using the variable name str in python. There is a function named str already:

import re
str_old = s = '$x start $x bar$xy $$x end $x'

str = re.sub(r'(\s|^)\$x(\s|$)' , r'\1$Q\2', str)

print(str)
print(str_old)

If you don't want to use the backrefereces, you can use lookarounds:

import re
str_old = s = '$x start $x bar$xy $$x end $x'  # You can remove this semicolon

str = re.sub(r'(?:(?<=\s)|(?<=^))\$x(?=\s|$)' , '$Q', str)
           # Since you don't have backreferences, you can now drop the rawing.

print(str)
print(str_old)

(?:(?<=\s)|(?<=^)) makes sure that the $x is preceded by a space or the beginning of the string;

(?=\s|$) makes sure that the $x is followed by a space or the end of the string.

edited Mar 29, 2014 at 16:29

answered Mar 29, 2014 at 11:48

Jerry

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3 Comments

Jerry Over a year ago

@xfrog Since you want it to work for $x $x, use the last code block here.

Tomas Bruckner Over a year ago

thank you, it works. I just had to change it a little to: re.sub(r'(?:(?<=\s)|(?<=^))(\$x)(?=\s|$)' , '$Q', s) because of "look-behind requires fixed-width pattern" error, now it works correctly. Thank you very much! :)

Jerry Over a year ago

@xfrog Oh, you're right! Forgot about that with python.

jfs · Accepted Answer · 2014-03-29 11:36:50Z

0

>>> import re
>>> re.sub(r'(\s|^)(\$x)(\s|$)', r'\1$Q\3', '$x start $x bar$xy $$x end $x')
'$Q start $Q bar$xy $$x end $Q'

answered Mar 29, 2014 at 11:36

jfs

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2 Comments

Tomas Bruckner Over a year ago

Thank you, this is exactly what I was looking for, but I just realised that the original regex won't work with string like this "$x $x" (only the first $x will sub). Isn't there a nice way how to correct it?

jfs Over a year ago

@xfrog: describe in plain English what you would like your regex to do, provide example input/output, your current attempt (a complete minimal example): what happens? what would you like to happen instead? And ask it as a new question

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