Find out how many times a regex matches in a string in Python

Question

Is there a way that I can find out how many matches of a regex are in a string in Python? For example, if I have the string "It actually happened when it acted out of turn."

I want to know how many times "ta" appears in the string. In that string, "ta" appears twice. I want my function to tell me it appeared twice. Is this possible?

Answer 1

import re
len(re.findall(pattern, string_to_search))

Answer 2

The existing solutions based on findall are fine for non-overlapping matches (and no doubt optimal except maybe for HUGE number of matches), although alternatives such as sum(1 for m in re.finditer(thepattern, thestring)) (to avoid ever materializing the list when all you care about is the count) are also quite possible. Somewhat idiosyncratic would be using subn and ignoring the resulting string...:

def countnonoverlappingrematches(pattern, thestring):
  return re.subn(pattern, '', thestring)[1]

the only real advantage of this latter idea would come if you only cared to count (say) up to 100 matches; then, re.subn(pattern, '', thestring, 100)[1] might be practical (returning 100 whether there are 100 matches, or 1000, or even larger numbers).

Counting overlapping matches requires you to write more code, because the built-in functions in question are all focused on NON-overlapping matches. There's also a problem of definition, eg, with pattern being 'a+' and thestring being 'aa' , would you consider this to be just one match, or three (the first a , the second one, both of them), or...?

Assuming for example that you want possibly-overlapping matches starting at distinct spots in the string (which then would give TWO matches for the example in the previous paragraph):

def countoverlappingdistinct(pattern, thestring):
  total = 0
  start = 0
  there = re.compile(pattern)
  while True:
    mo = there.search(thestring, start)
    if mo is None: return total
    total += 1
    start = 1 + mo.start()

Note that you do have to compile the pattern into a RE object in this case: function re.search does not accept a start argument (starting position for the search) the way method search does, so you'd have to be slicing thestring as you go -- definitely more effort than just having the next search start at the next possible distinct starting point, which is what I'm doing in this function.

Answer 3

I know this is a question about regex. I just thought I'd mention the count method for future reference if someone wants a non-regex solution.

>>> s = "It actually happened when it acted out of turn."
>>> s.count('t a')
2

Which return the number of non-overlapping occurrences of the substring

Answer 4

You can find overlapping matches by using a noncapturing subpattern:

def count_overlapping(pattern, string):
    return len(re.findall("(?=%s)" % pattern, string))

Answer 5

你试过这个吗？

 len( pattern.findall(source) )

Answer 6

import re
print len(re.findall(r'ab',u'ababababa'))

Answer 7

To avoid creating a list of matches one may also use re.sub with a callable as replacement. It will be called on each match, incrementing internal counter.

class Counter(object):
    def __init__(self):
        self.matched = 0
    def __call__(self, matchobj):
        self.matched += 1

counter = Counter()
re.sub(some_pattern, counter, text)

print counter.matched

Answer 8

this works fine

ptr_str = lambda pattern,string1 :print(f'pattern = {pattern} times = {len(re.findall(pattern,string1))}')
pattern = 'AGATC'
str='AAGGTAAGTTTAGAATATAAAAGGTGAGTTAAATAGATCATAGGTTATATTGT'
ptr_str(pattern,string1)