In Python, how to check if a string only contains certain characters?

by

Here’s a simple, pure-Python implementation. It should be used when performance is not critical (included for future Googlers).

import string
allowed = set(string.ascii_lowercase + string.digits + '.')

def check(test_str):
    set(test_str) <= allowed

Regarding performance, iteration will probably be the fastest method. Regexes have to iterate through a state machine, and the set equality solution has to build a temporary set. However, the difference is unlikely to matter much. If performance of this function is very important, write it as a C extension module with a switch statement (which will be compiled to a jump table).

Here’s a C implementation, which uses if statements due to space constraints. If you absolutely need the tiny bit of extra speed, write out the switch-case. In my tests, it performs very well (2 seconds vs 9 seconds in benchmarks against the regex).

#define PY_SSIZE_T_CLEAN
#include <Python.h>

static PyObject *check(PyObject *self, PyObject *args)
{
        const char *s;
        Py_ssize_t count, ii;
        char c;
        if (0 == PyArg_ParseTuple (args, "s#", &s, &count)) {
                return NULL;
        }
        for (ii = 0; ii < count; ii++) {
                c = s[ii];
                if ((c < '0' && c != '.') || c > 'z') {
                        Py_RETURN_FALSE;
                }
                if (c > '9' && c < 'a') {
                        Py_RETURN_FALSE;
                }
        }

        Py_RETURN_TRUE;
}

PyDoc_STRVAR (DOC, "Fast stringcheck");
static PyMethodDef PROCEDURES[] = {
        {"check", (PyCFunction) (check), METH_VARARGS, NULL},
        {NULL, NULL}
};
PyMODINIT_FUNC
initstringcheck (void) {
        Py_InitModule3 ("stringcheck", PROCEDURES, DOC);
}

Include it in your setup.py:

from distutils.core import setup, Extension
ext_modules = [
    Extension ('stringcheck', ['stringcheck.c']),
],

Use as:

>>> from stringcheck import check
>>> check("abc")
True
>>> check("ABC")
False

More Related Contents:

  1. What is the difference between re.search and re.match?
  2. How can I find all matches to a regular expression in Python?
  3. How to replace the first occurrence of a regular expression in Python?
  4. python – regex search and findall
  5. Take first some character of string but if particular character will come consider that character also
  6. Extract part of a regex match
  7. Regular expression matching a multiline block of text
  8. Is there a short contains function for lists?
  9. How can I do multiple substitutions using regex?
  10. regex error – nothing to repeat
  11. How to grab number after word in python
  12. Extract a string between double quotes
  13. Find the indexes of all regex matches?
  14. RegEx Get string between two strings that has line breaks
  15. Most efficient way for a lookup/search in a huge list (python)
  16. Python – re.findall returns unwanted result
  17. Why Does a Repeated Capture Group Return these Strings?
  18. Splitting out the output of ps using Python
  19. How to use regex to find all overlapping matches
  20. How can a recursive regexp be implemented in python?
  21. Fastest way to search a list in python
  22. Python regular expression pattern * is not working as expected
  23. Split string at every position where an upper-case word starts
  24. FutureWarning: The default value of regex will change from True to False in a future version
  25. Why does this take so long to match? Is it a bug?
  26. Find starting and ending indices of sublist in list
  27. Regex matching – why does this not match and return None?
  28. How to use str.contains() with multiple expressions in pandas dataframes
  29. Searching a sorted list? [closed]
  30. How to use regex with optional characters in python?

Leave a Comment