How to use 'python program to count the number of vowels in a string' in Python

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def count_letters(string):
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    freq, limit, i = {}, len(string), 0
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    while i < limit:
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        if string[i] in freq:
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            freq[string[i]] += 1
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        else:
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            freq[string[i]] = 1
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        i += 1
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    return freq

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def char_count(w):
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    num = 0
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    for i in xrange(len(w)):
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        if re.search(r'[a-zA-Z]', w[i]):
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            num += 1
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    return num