Edit - Stack Overflow

You are not logged in. Your edit will be placed in a queue until it is peer reviewed.

We welcome edits that make the post easier to understand and more valuable for readers. Because community members review edits, please try to make the post substantially better than how you found it, for example, by fixing grammar or adding additional resources and hyperlinks.

Required fields*

Rev

Required fields*

python unicode string in regexes

I have an unicode encoded (with BOM) source file and some string that contains unicode symbols. I want to replace all characters that not belong to a defined character set with an underscore.

# coding: utf-8 
import os
import sys
import re
t = "🙂 [°] \n € dsf $ ¬ 1 Ä 2 t34円Ú";
print re.sub(r'[^A-Za-z0-9 !#%&()*+,-./:;<=>?[\]^_{|}~"\'\\]', '_', t, flags=re.UNICODE)
output: ____ [__] _ ___ dsf _ __ 1 __ 2 t3__4__
expected: _ [_] _ _ dsf _ _ 1 _ 2 t3_4_

But each character is replaced by a number of its underscores that may be equal to the bytes in its unicode representation.

Maybe an additional problem:

In the actual problem the strings is read from a unicode file by another python module and I do not know if it handles the unicodeness correctly. So may be the string variable is marked as ascii but contains unicode sequences.

Answer*

Operate on Unicode strings, not byte strings. Your source is encoded as UTF-8 so the characters are encoded from one to four bytes each. Decoding to Unicode strings or using Unicode constants will help. The code also appears to be Python 2-based, so on narrow Python 2 builds (the default on Windows) you'll still have an issue. You could also have issues if you have graphemes built with two or more Unicode code points:

 # coding: utf-8 
 import re
 
 t = u"🙂 [°] \n € dsf $ ¬ 1 Ä 2 t3¥4Ú";
 print re.sub(ur'[^A-Za-z0-9 !#%&()*+,-./:;<=>?[\]^_{|}~"\'\\]', '_', t, flags=re.UNICODE)

Output (on Windows Python 2.7 narrow build):

 __ [_] _ _ dsf _ _ 1 _ 2 t3_4_

Note the first emoji still has a double-underscore. Unicode characters greater than U+FFFF are encoded as surrogate pairs. This could be handled by explicitly checking for them. The first code point of a surrogate pair is U+D800 to U+DBFF and the second is U+DC00 to U+DFFF:

 # coding: utf-8 
 import re
 
 t = u"🙂 [°] \n € dsf $ ¬ 1 Ä 2 t3¥4Ú";
 print re.sub(ur'[\ud800-\udbff][\udc00-\udfff]|[^A-Za-z0-9 !#%&()*+,-./:;<=>?[\]^_{|}~"\'\\]', '_', t, flags=re.UNICODE)

Output:

 _ [_] _ _ dsf _ _ 1 _ 2 t3_4_

But you'll still have a problem with complex emoji:

 # coding: utf-8 
 import re
 
 t = u"👨🏻‍👩🏻‍👧🏻‍👦🏻";
 print re.sub(ur'[\ud800-\udbff][\udc00-\udfff]|[^A-Za-z0-9 !#%&()*+,-./:;<=>?[\]^_{|}~"\'\\]', '_', t, flags=re.UNICODE)

Output:

 ___________

Draft saved

Draft discarded

Edit Summary*

Cancel

Your string with complex emoji is composed of 11 characters, and 11 underscores are returned.

Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams

2018年05月08日 00:10:38 +00:00
Commented May 8, 2018 at 0:10
@IgnacioVazquez-Abrams hence the problem. Visually the complex emoji looks like one character (or four, on my current browser). Not an easy problem to solve. The 3rd party regex module can detect graphemes, but I'm just pointing out the problem is still complex.

Mark Tolonen
– Mark Tolonen

2018年05月08日 13:59:27 +00:00
Commented May 8, 2018 at 13:59
thanks, but what if (my addition at the end) the string is defined somewhere and is not a unicode string but a normal string that was read in without proper encoding set. Ho can i treat this string (that is mor kind of a byte array) as an unicde string?

vlad_tepesch
– vlad_tepesch

2018年05月09日 06:27:46 +00:00
Commented May 9, 2018 at 6:27
@vlad_tepesch .decode() the string with the correct encoding.

Mark Tolonen
– Mark Tolonen

2018年05月09日 06:59:57 +00:00
Commented May 9, 2018 at 6:59

Add a comment |

How to Edit

Correct minor typos or mistakes
Clarify meaning without changing it
Add related resources or links
Always respect the author’s intent
Don’t use edits to reply to the author

How to Format

create code fences with backticks ` or tildes ~
```
like so
```
add language identifier to highlight code
```python
def function(foo):
print(foo)
```
put returns between paragraphs
for linebreak add 2 spaces at end
_italic_ or **bold**
indent code by 4 spaces
backtick escapes `like _so_`
quote by placing > at start of line
to make links (use https whenever possible)

<https://example.com>

[example](https://example.com)

<a href="https://example.com">example</a>

formatting help »
answering help »

How to Tag

A tag is a keyword or label that categorizes your question with other, similar questions. Choose one or more (up to 5) tags that will help answerers to find and interpret your question.

complete the sentence: my question is about...
use tags that describe things or concepts that are essential, not incidental to your question
favor using existing popular tags
read the descriptions that appear below the tag

If your question is primarily about a topic for which you can't find a tag:

combine multiple words into single-words with hyphens (e.g. python-3.x), up to a maximum of 35 characters
creating new tags is a privilege; if you can't yet create a tag you need, then post this question without it, then ask the community to create it for you

popular tags »

lang-py

CollectivesTM on Stack Overflow

python unicode string in regexes

Answer*