Question

When I use the following python regex to perform the functionality described below, I get the error Unexpected end of Pattern.

Regex:

modified=re.sub(r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(?-i)
(CODE[0-9]{3})(?!</a>)',r'<a href="http://productcode/\g<1>">\g<1></a>',input)

Purpose of this regex:

INPUT:

CODE876
CODE223
matchjustCODE657
CODE69743
code876
testing1CODE888
example2CODE098
http://replaced/CODE665

Should match:

CODE876
CODE223
CODE657
CODE697

and replace occurrences with

http://productcode/CODE876
http://productcode/CODE223
matchjusthttp://productcode/CODE657
http://productcode/CODE69743

Should Not match:

code876
testing1CODE888
testing2CODE776
example3CODE654
example2CODE098
http://replaced/CODE665

FINAL OUTPUT

http://productcode/CODE876
http://productcode/CODE223
matchjusthttp://productcode/CODE657
http://productcode/CODE69743
code876
testing1CODE888
example2CODE098
http://replaced/CODE665

EDIT and UPDATE 1

modified=re.sub(r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(CODE[0-9]{3})(?!</a>)',r'<a href="http://productcode/\g<1>">\g<1></a>',input)

The error is no more happening. But this does not match any of the patterns as needed. Is there a problem with matching groups or the matching itself. Because when I compile this regex as such, I get no match to my input.

EDIT AND UPDATE 2

f=open("/Users/mymac/Desktop/regex.txt")
s=f.read()

s1 = re.sub(r'((?!http://|testing[0-9]|example[0-9]).*?)(CODE[0-9]{3})(?!</a>)', 
            r'\g<1><a href="http://productcode/\g<2>">\g<2></a>', s)
print s1

INPUT

CODE123 CODE765 testing1CODE123 example1CODE345 http://www.coding.com/CODE333 CODE345

CODE234

CODE333

OUTPUT

<a href="http://productcode/CODE123">CODE123</a> <a href="http://productcode/CODE765">CODE765</a> testing1<a href="http://productcode/CODE123">CODE123</a> example1<a href="http://productcode/CODE345">CODE345</a> http://www.coding.com/<a href="http://productcode/CODE333">CODE333</a> <a href="http://productcode/CODE345">CODE345</a>

<a href="http://productcode/CODE234">CODE234</a>

<a href="http://productcode/CODE333">CODE333</a>

Regex works for Raw input, but not for string input from a text file.

See Input 4 and 5 for more results http://ideone.com/3w1E3

Answer 1

Okay, it looks like the problem is the (?-i), which is surprising. The purpose of the inline-modifier syntax is to let you apply modifiers to selected portions of the regex. At least, that’s how they work in most flavors. In Python it seems they always modify the whole regex, same as the external flags (re.I, re.M, etc.). The alternative (?i:xyz) syntax doesn’t work either.

On a side note, I don’t see any reason to use three separate lookaheads, as you did here:

(?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?

Just OR them together:

(?:(?!http://|testing[0-9]|example[0-9]).)*?

---

EDIT: We seem to have moved from the question of why the regex throws exceptions, to the question of why it doesn’t work. I’m not sure I understand your requirements, but the regex and replacement string below return the results you want.

s1 = re.sub(r'^((?!http://|testing[0-9]|example[0-9]).*?)(CODE[0-9]{3})(?!</a>)', 
            r'\g<1><a href="http://productcode/\g<2>">\g<2></a>', s)

see it in action one ideone.com

Is that what you’re after?

---

EDIT: We now know that the replacements are being done within a larger text, not on standalone strings. That’s makes the problem much more difficult, but we also know the full URLs (the ones that start with http://) only occur in already-existing anchor elements. That means we can split the regex into two alternatives: one to match complete <a>...</a> elements, and one to match our the target strings.

(?s)(?:(<a\s+[^>]*>.*?</a>)|\b((?:(?!testing[0-9]|example[0-9])\w)*?)(CODE[0-9]{3}))

The trick is to use a function instead of a static string for the replacement. Whenever the regex matches an anchor element, the function will find it in group(1) and return it unchanged. Otherwise, it uses group(2) and group(3) to build a new one.

here’s another demo (I know that’s horrible code, but I’m too tired right now to learn a more pythonic way.)