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Regex to remove strings from list that do not match given prefix

I have a string that includes multiple comma-separated lists of values, always embedded between <mks:Field name="MyField"> and </mks:Field>.
For example:
<mks:Field name="MyField">X001_ABC</mks:Field><mks:Field name="AnotherField">X002_XYZ</mks:Field><mks:Field name="MyField"></mks:Field><mks:Field name="MyField">X000_Test1,X000_Test2</mks:Field><mks:Field name="MyField">X001_ABC,X000_Test1</mks:Field><mks:Field name="MyField">X000_Test1,X000_Test2,X002_XYZ</mks:Field>
In this example I have the following values to work with:
X001_ABC
(empty)
X000_Test1,X000_Test2
X001_ABC,X000_Test1
X000_Test1,X000_Test2,X002_XYZ
Now I want to remove all the values that do not start with the prefix ""X000_", including any needless commas, so that my result looks like this:
<mks:Field name="MyField"></mks:Field><mks:Field name="AnotherField">X002_XYZ</mks:Field><mks:Field name="MyField"></mks:Field><mks:Field name="MyField">X000_Test1,X000_Test2</mks:Field><mks:Field name="MyField">X000_Test1</mks:Field><mks:Field name="MyField">X000_Test1,X000_Test2</mks:Field>
I have tried the following regex, but it does not work properly if only one value exists not matching my regex and I do not want to change my regex if a new value matching my prefix is introduced (e.g. X000_Test3).
Search: (?<=name="MyField">)[^<>](?:.*?(X000_Test1,X000_Test2|X000_Test1|X000_Test2))?.*?(?=</mks:Field>)
Replace: \1
This gives me the following result that does not match the expected output:
<mks:Field name="MyField">X000_Test1,X000_Test2</mks:Field><mks:Field name="MyField">X000_Test1</mks:Field><mks:Field name="MyField">X000_Test2</mks:Field>
Unfortunately I cannot simply parse the string with something else - I only have the option of a regex search/replace in this case.
Thank you in advance, any help would be appreciated.

If you are using Javascript use this:
prefix='X000';
let pattern= new RegExp(`((?<=>)|,)((?!${prefix}|[>\<,]).)*(,|(?=\<))`, 'g');
For any other language use this:
'/((?<=>)|,)((?!X000|[>\<,]).)*(,|(?=\<))/';
X000 being the prefix you want to keep

I can't use properly the replace command in python

I'm new on the platform:
I have a doubt about the use of the replace command.
Here is my code and what I need to do:
The thing is that I need to replace a substring with a string sequence.
For example:
from: Hitacoworld to Hiworld.
The thing is that I part from a dictionary with the string and substrings and I don't know how to get the string without the substring part. I use the command replace this way:
ntdna = ''
ntdna += string.replace(seqs[element],'',)
Where 'string' is my string and 'seqs[element]' are the substrings that I want to remove from the string, but the problem is that when I see the outcome I'm not getting the right string, so I think the problem is in the use of the replace, any hint or something I could use? Ty in advance, I'm lost.
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How to split a string on multiple pattern using pythonic way (one liner)?

I am trying to extract file name from file pointer without extension. My file name is as follows:
this site:time.list,this.list,this site:time_sec.list, that site:time_sec.list and so on. Here required file name always precedes either whitespace or dot.
Currently I am doing this to get file from file name preceding white space and dot in file name.
search_term = os.path.basename(f.name).split(" ")[0]
and
search_term = os.path.basename(f.name).split(".")[0]
Expected file name output: this, this, this, that.
How can i combine above two into one liner kind and pythonic way?
Thanks in advance.

using regex as below,
[ .] will split either on a space or a dot char
re.split('[ .]', os.path.basename(f.name))[0]

If you split on one and splitting on the other still returns something smaller, that's the one you want. If not, what you get is what you got from the first split. You don't need regex for this.
search_term = os.path.basename(f.name).split(" ")[0].split(".")[0]

Use regex to get the first word at the beginning of the string:
import re
re.match(r"\w+", "this site:time_sec.list").group()
# 'this'
re.match(r"\w+", "this site:time.list").group()
# 'this'
re.match(r"\w+", "that site:time_sec.list").group()
# 'that'
re.match(r"\w+", "this.list").group()
# 'this'
try this:
pattern = re.compile(r"\w+")
pattern.match(os.path.basename(f.name)).group()
Make sure your filenames don't have whitespace inside when you rely on the assumption that a whitespace separates what you want to extract from the rest. It's much more likely to get unexpected results you didn't think up in advance if you rely on implicit rules like that instead of actually looking at the strings you want to extract and tailor explicit expressions to fit the content.

Python - Injecting html tags into strings based on regex match

I wrote a script in Python for custom HTML page that finds a word within a string/line and highlights just that word with use of following tags where instance is the word that is searched for.
<b><font color=\"red\">"+instance+"</font></b>
With the following result:
I need to find a word (case insensitive) let's say "port" within a string that can be port, Port, SUPPORT, Support, support etc, which is easy enough.
pattern = re.compile(word, re.IGNORECASE)
find_all_instances = pattern.findall(string_to_search)
However my strings often contain 2 or more instances in single line, and I need to append
<b><font color=\"red\">"+instance+"</font></b> to each of those instances, without changing cases.
Problem with my approach, is that I am attempting to itterate over each of instances found with findall (exact match),
while multiple same matches can also be found within the string.
for instance in find_all_instances:
second_pattern = re.compile(instance)
string_to_search = second_pattern.sub("<b><font color=\"red\">"+instance+"</font></b>", string_to_search)
This results in following:
<b><font color="red"><b><font color="red"><b><font color="red">Http</font></b></font></b></font></b></font>
when I need
<b><font color="red">Http</font></b>
I was thinking, I would be able to avoid this if I was able to find out exact part of the string that the pattern.sub substitutes at the moment of doing it,
however I was not able to find any examples of that kind of usage, which leads me to believe that I am doing something very wrong.
If anyone have a way I could use to insert <b><font color="red">instance</font></b> without replacing instance for all matches(case insensitive), then I would be grateful.

Maybe I'm misinterpretting your question, but wouldn't re.sub be the best option?
Example: https://repl.it/DExs

Okay so two ways I did quickly! The second loop is definitely the way to go. It uses re.sub (as someone else commented too). It replaces with the lowercase search term bear in mind.
import re
FILE = open("testing.txt","r")
word="port"
#THIS LOOP IS CASE SENSITIVE
for line in FILE:
newline=line.replace(word,"<b><font color=\"red\">"+word+"</font></b>")
print newline
#THIS LOOP IS INCASESENSITIVE
for line in FILE:
pattern=re.compile(word,re.IGNORECASE)
newline = pattern.sub("<b><font color=\"red\">"+word+"</font></b>",line)
print newline

Replacing strings in a text and ignoring certain parts

I found many programs online to replace text in a string or file with words prescribed in a dictionary. For example, https://www.daniweb.com/programming/software-development/code/216636/multiple-word-replace-in-text-python
But I was wondering how to get the program to ignore certain parts of the text. For instance, I would like it to ignore parts that are ensconced within say % signs (%Please ignore this%). Better still, how do I get it to ignore the text within but remove the % sign at the end of the run.
Thank you.

This could very easily be done with regular expressions, although they may not be supported by any online programs you find. You will probably need to write something yourself and then use regex as your dict's search key's.
Good place to start playing around with regex is: http://regexr.com

Well in the replacing dictionary just have any word you want to be ignored such as teh be replaced with the but %teh% be replaced with teh. For the program in the link you could have
wordDic = {
'booster': 'rooster',
'%booster%': 'booster'
}