在文本文件中查找包含特定字符且具有特定长度的单词

Question

I am trying to find words within a text file that are of 7 letters in length and contain the letters a, b, c, e and r. So far I have this:

import re

file = open("dictionary.txt","r")
text = file.readlines()
file.close()


keyword = re.compile(r'\w{7}')

for line in text:
    result = keyword.search (line)
    if result:
       print (result.group())

Can anyone help me?

Answer 1

You need to not only match the word characters, but also the word boundary :

keyword = re.compile(r'\b\w{7}\b')

The \\b anchor matches at the start or end of a word, limiting the word to exactly 7 characters.

It'd be more efficient if you were to loop through the file line-by-line instead of reading it all into memory in one go:

import re

keyword = re.compile(r'\b\w{7}\b')

with open("dictionary.txt","r") as dictionary:    
    for line in dictionary:
        for result in keyword.findall(line):
            print(result)

Using keyword.findall() gives us a list of all matches on the line.

To check if the matches have at least one of the required characters in it, I personally would just use a set intersection test:

import re

keyword = re.compile(r'\b\w{7}\b')
required = set('abcer')

with open("dictionary.txt","r") as dictionary:    
    for line in dictionary:
        results = [required.intersection(word) for word in keyword.findall(line)]
        for result in results
            print(result)

Answer 2

\b(?=\w{0,6}?[abcer])\w{7}\b

That's the regular expression you want. It works by using the basic form for a word of exactly seven letters ( \\b\\w{7}\\b ) and adding a lookahead - a zero width assertion that looks forward and tries to find one of your required letters. A breakdown:

\b            A word boundary
(?=           Look ahead and find...
    \w        A word character (A-Za-z0-9_)
    {0,6}     Repeated 0 to 6 times
    ?         Lazily (not necessary, but marginally more efficient).
    [abcer]   Followed by one of a, b, c, e, or r
)             Go back to where we were before (just after the word boundary
\w            And match a word character
{7}           Exactly seven times.
\b            Then one more word Boundary.