Stripping everything but alphanumeric chars from a string in Python

Use the str.translate() method.

Presuming you will be doing this often:

1. Once, create a string containing all the characters you wish to delete:

   delchars = ''.join(c for c in map(chr, range(256)) if not c.isalnum())
   

2. Whenever you want to scrunch a string:

   scrunched = s.translate(None, delchars)
   

The setup cost probably compares favourably with re.compile; the marginal cost is way lower:

C:\junk>\python26\python -mtimeit -s"import string;d=''.join(c for c in map(chr,range(256)) if not c.isalnum());s=string.printable" "s.translate(None,d)"
100000 loops, best of 3: 2.04 usec per loop

C:\junk>\python26\python -mtimeit -s"import re,string;s=string.printable;r=re.compile(r'[\W_]+')" "r.sub('',s)"
100000 loops, best of 3: 7.34 usec per loop

Note: Using string.printable as benchmark data gives the pattern '[\W_]+' an unfair advantage; all the non-alphanumeric characters are in one bunch ... in typical data there would be more than one substitution to do:

C:\junk>\python26\python -c "import string; s = string.printable; print len(s),repr(s)"
100 '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!"#$%&\'()*+,-./:;=>?@[\\]^_`{|}~ \t\n\r\x0b\x0c'

Here's what happens if you give re.sub a bit more work to do:

C:\junk>\python26\python -mtimeit -s"d=''.join(c for c in map(chr,range(256)) if not c.isalnum());s='foo-'*25" "s.translate(None,d)"
1000000 loops, best of 3: 1.97 usec per loop

C:\junk>\python26\python -mtimeit -s"import re;s='foo-'*25;r=re.compile(r'[\W_]+')" "r.sub('',s)"
10000 loops, best of 3: 26.4 usec per loop

I just timed some functions out of curiosity. In these tests I'm removing non-alphanumeric characters from the string string.printable (part of the built-in string module). The use of compiled '[\W_]+' and pattern.sub('', str) was found to be fastest.

$ python -m timeit -s \
     "import string" \
     "''.join(ch for ch in string.printable if ch.isalnum())" 
10000 loops, best of 3: 57.6 usec per loop

$ python -m timeit -s \
    "import string" \
    "filter(str.isalnum, string.printable)"                 
10000 loops, best of 3: 37.9 usec per loop

$ python -m timeit -s \
    "import re, string" \
    "re.sub('[\W_]', '', string.printable)"
10000 loops, best of 3: 27.5 usec per loop

$ python -m timeit -s \
    "import re, string" \
    "re.sub('[\W_]+', '', string.printable)"                
100000 loops, best of 3: 15 usec per loop

$ python -m timeit -s \
    "import re, string; pattern = re.compile('[\W_]+')" \
    "pattern.sub('', string.printable)" 
100000 loops, best of 3: 11.2 usec per loop

Regular expressions to the rescue:

import re
re.sub(r'\W+', '', your_string)

By Python definition '\W == [^a-zA-Z0-9_], which excludes all numbers, letters and _