Home/ Questions/Q 6477189
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-25T06:55:04+00:00

I’m not so able with regex and I’m looking for the syntax to exclude

  • 0

I’m not so able with regex and I’m looking for the syntax to exclude something.
I’m parsing <, >, " and & in html code (to replace with &lt;, etc) and I need to exclude <br/> from parsing.
I.E.:

<html><br/>
   <head><title></title></head><br/>
   <body><br/>
   </body><br/>
</html>

I tried sometihng like i.e.: r'<\b?![br]' and others, but they don’t work completely. I use re.sub() to replace.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    Ok, now the question is open again, I can do it as an answer, so…

    Unless I’m missing something, and once it’s just <br/> (not any variants), then can just replace <(?!br/>) with &lt; and (?<!<br/)> with &gt; and that’s it?

    In Python, it looks like that means this:

    text = re.sub( '<(?!br/>)' , '&lt;' , text )
text = re.sub( '(?<!<br/)>' , '&gt;' , text )

    To explain what’s going on, (?!…) is a negative lookahead – it only successfully matches at a position if the following text does not match the sub-expression it contains.
    (Note lookaheads do not consume the text matched by their sub-expression, they only verify if it exists, or not.)

    Similarly, (?<!…) is a negative lookbehind, and does the same thing but using the preceding text.

    However, lookbehinds do have a slight different to lookaheads (in some regex implementations) – which is that the sub-expressions inside lookbehinds must represent fixed-width or limited-width matches.

    Python is one of the ones that requires a fixed width – so whilst the above expression works (because it’s always four characters), if it was (?<!<br\s*/?)> then it would not be a valid regex for Python because it represents a variable length match. (However, you can stack multiple lookbehinds, so you could potentially manually iterate the assorted options, if that was necessary.)

    • 0