On complex regex, remember this:

First, you must put \ before any ' in your regex. otherwise it will give conflict with the Python syntax that also have the operator '

You had to remove the r after find : part. So, should be 'foo' instead of r'foo'

In your default example should be (without : r in front of 'foo'

{ 'find' : 'foo', 'replace' : r'bar' },

Your code, with the new and complex regex, should be like this.

I write a second regex to see much better:

# -*- coding: utf-8 -*-
from __future__ import print_function

from Npp import *
import os
import sys

#-------------------------------------------------------------------------------

class T22601(object):

    def __init__(self):
        Path="C:\\python-test"
        for root, dirs, files in os.walk(Path):
            nested_levels = root.split('/')
            if len(nested_levels) > 0:
                del dirs[:]
            for filename in files:		
                if filename[-5:] == '.html':
                    notepad.open(root + "\\" + filename)
                    console.write(root + "\\" + filename + "\r\n")
                    notepad.runMenuCommand("Encodage", "Convertir en UTF-8")
                    regex_find_repl_dict_list = [
                        { 'find' : '<link (.*?)( rel=\'canonical\'/>)', 'replace' : r'<link rel="canonical" \1 />' },
                        { 'find' : '(<meta content=\')(.*)(\' property=\'og:description\'/>)', 'replace' : r'<meta name="description" content="\2" />' },
                    ]
                    editor.beginUndoAction()
                    for d in regex_find_repl_dict_list: editor.rereplace(d['find'], d['replace'])
                    editor.endUndoAction()
                    notepad.save()
                    notepad.close()
#-------------------------------------------------------------------------------

if __name__ == '__main__': T22601()

Don’t use single quotes inside single quotes, or double quotes inside double quotes

Do use double quotes inside single quotes, or single quotes inside double quotes

something like: flags=re.MULTILINE

{ 'find' : r'<link (.*?)( rel='canonical'/>)', 'replace' : r'<link rel="canonical" \1 />' }, flags=re.MULTILINE