RE Question

[Gabriel Genellina]

En Sun, 02 Aug 2009 18:22:20 -0300, Victor Subervi  
<victorsubervi at gmail.com> escribió:

> How do I search and replace something like this:
> aLine = re.sub('[<]?[p]?[>]?<font size="h' + str(x) + '"[
> a-zA-Z0-9"\'=:]*>[<]?[b]?[>]?', '<h' + str(x) + '>', aLine)
> where RE *only* looks for the possibility of "<p>" at the beginning of  
> the
> string; that is, not the individual components as I have it coded above,  
> but
> the entire 3-character block?

An example would make it more clear; I think you want to match either  
"<p><font size=...." or "<font size=....". In other words, "<p>" is  
optional. Use a normal group or a non-capturing group:
r'(<p>)?<font size="...'
r'(?:<p>)?<font size="...'

That said, using regular expressions to parse HTML or XML is terribly  
fragile; I'd use a specific tool (like BeautifulSoup, ElementTree, or lxml)

-- 
Gabriel Genellina