How to use variable in python regex?

Question

I am trying to process user input in regex from variable. After a lot of searching I have come up with following:

Explaination of code variables:

step is a string used as input for regex

e.g.

replace|-|space ,

replace|*|null,

replace|/|\|squot|space

b is a list of elements. Element is fetched and modified as per regex.

i is integer received from other function to access list b using i as index

I process the above string to get array, then use the last element of array as substitution string

First element is deleted as it is not required. All other elements need to be replaced with substitution string.

def replacer(step,i,b):
    steparray = step.split('|')
    del steparray[0]
    final = steparray.pop()

    if final == "space":
        subst = u" "
    elif final == "squot":
        subst = u"'"
    elif final == "dquot":
        subst = u"\""
    else:
        subst = u"%s"%final

    for input in xrange(0,len(steparray)):
        test=steparray[input]
        regex = re.compile(ur'%s'%test)
        b[i] = re.sub(regex, subst, b[i])
    print b[i]

However, when I run above code, following error is shown:

  File "CSV_process.py", line 78, in processor
    replacer(step,i,b)
  File "CSV_process.py", line 115, in replacer
    regex = re.compile(ur'%s'%test)
  File "/usr/lib/python2.7/re.py", line 190, in compile
    return _compile(pattern, flags)
  File "/usr/lib/python2.7/re.py", line 242, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat

I tried a lot but dont understand how regex works. Please help with error.

Final requirement is to get a special character from user input and replace it with another character (again from user input)

PS: Also, the code does not have 242 lines but error is on line 242. Is the error occurring after end of array in for loop?

What do you mean by it begins with *? I have not allowed * as input but the b[i] may contain * — AJINKYA
– AJINKYA, Commented Feb 8, 2015 at 9:37

falsetru · Accepted Answer · 2015-02-08 09:31:26Z

3

Some special characters like * should be escaped to match literally.

>>> import re
>>> re.compile('*')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python27\lib\re.py", line 194, in compile
    return _compile(pattern, flags)
  File "C:\Python27\lib\re.py", line 251, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat

Using re.escape, you can escape them:

>>> print(re.escape('*'))
\*
>>> re.compile(re.escape('*'))
<_sre.SRE_Pattern object at 0x000000000273DF10>

BTW, if you want to simply replace them, regular expression is not necessary. Why don't you use str.replace?

replaced_string = string_object.replace(old, new)

answered Feb 8, 2015 at 9:31

falsetru

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2 Comments

AJINKYA Over a year ago

Will str.replace replace all occurrences of particular character? Sorry for silly questions, just started coding in python.

falsetru Over a year ago

@AJINKYA, Yes it does. You can control how many replacement will be happen. Try '123123'.replace('2', '.') and '123123'.replace('2', '.', 1).

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