CODEWITHSUNDEEP
pythonregexpattern-matching
c_prog_90
c_prog_90

Reputation: 949

Unexpected end of Pattern : Python Regex

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

Upvotes: 1

Views: 7335

Answers (4)

Alan Moore
Alan Moore

Reputation: 75222

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.)

Upvotes: 2

John Machin
John Machin

Reputation: 82924

Your main problem is the (?-i) thingy which is wishful thinking as far as Python 2.7 and 3.2 are concerned. For more details, see below.

import re
# 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)
# observation 1: as presented, pattern has a line break in the middle, just after (?-i)
# ob 2: rather hard to read, should use re.VERBOSE
# ob 3: not obvious whether it's a complile-time or run-time problem
# ob 4: (?i) should be at the very start of the pattern (see docs)
# ob 5: what on earth is (?-i) ... not in 2.7 docs, not in 3.2 docs
pattern = r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(?-i)(CODE[0-9]{3})(?!</a>)'
#### rx = re.compile(pattern)
# above line failed with "sre_constants.error: unexpected end of pattern"
# try without the (?-i)
pattern2 = r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(CODE[0-9]{3})(?!</a>)'
rx = re.compile(pattern2)
# This works, now you need to work on observations 1 to 4,
# and rethink your CODE/code strategy

Looks like suggestions fall on deaf ears ... Here's the pattern in re.VERBOSE format:

pattern4 = r'''
    ^
    (?i)
    (
        (?:
            (?!http://)
            (?!testing[0-9])
            (?!example[0-9])
            . #### what is this for?
        )*?
    ) ##### end of capturing group 1
    (CODE[0-9]{3}) #### not in capturing group 1
    (?!</a>)
    '''

Upvotes: 5

Karl Knechtel
Karl Knechtel

Reputation: 61498

For complex regexes, use the re.X flag to document what you're doing and to make sure the brackets match up correctly (i.e. by using indentation to indicate the current level of nesting).

Upvotes: 0

stema
stema

Reputation: 92976

The only problem I see is that you replace using the wrong capturing group.

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)  
                       ^                                                        ^                                                        ^
                    first capturing group                                  second one                                         using the first group

Here I made the first one also a non capturing group

^(?i)(?:(?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(?-i)(CODE[0-9]{3})(?!</a>)

See it here on Regexr

Upvotes: 0

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