Question

I need to add “php” to all urls in href="xxx", that don’t end with “php”.
I use negetive lookahead (?!php):

find = r'href="(.+?)(?!php)"'
replace =  r'href="\1.php"'
re.sub(find, replace, 'href="url"')
re.sub(find, replace, 'href="url.php"')

both add extension:

href="url.php"
href="url.php.php"

Why negative lookahead doesn’t work?

Answer 1

The following does work:

In [49]: re.sub(r'href="([^"]*?)([.]php)?"', r'href="\1.php"', 'href="url.php"')
Out[49]: 'href="url.php"'

In [50]: re.sub(r'href="([^"]*?)([.]php)?"', r'href="\1.php"', 'href="url"')
Out[50]: 'href="url.php"'

The reason your original regex (.+?)(?!php) doesn’t quite work is that it matches url.php as follows:

• (.+?) matches url.php;
• at this point the negative lookahead is satisfied since the next character is a double quote.

In other words, .+? consumes the entire filename including the extension, making the lookahead a no-op.