Regex to find words that start or end with a particular letter

Question

Write a function called getWords(sentence, letter) that takes in a sentence and a single letter, and returns a list of the words that start or end with this letter, but not both, regardless of the letter case.

For example:

>>> s = "The TART program runs on Tuesdays and Thursdays, but it does not start until next week."
>>> getWords(s, "t")
['The', 'Tuesdays', 'Thursdays', 'but', 'it', 'not', 'start', 'next']

My attempt:

regex = (r'[\w]*'+letter+r'[\w]*')
return (re.findall(regex,sentence,re.I))

My Output:

['The', 'TART', 'Tuesdays', 'Thursdays', 'but', 'it', 'not', 'start', 'until', 'next']

Answer 1

\\b detects word breaks. Verbose mode allows multi-line regexs and comments. Note that [^\\W] is the same as \\w , but to match \\w except a certain letter, you need [^\\W{letter}] .

import re

def getWords(s,t):
    pattern = r'''(?ix)           # ignore case, verbose mode
                  \b{letter}      # start with letter
                  \w*             # zero or more additional word characters
                  [^{letter}\W]\b # ends with a word character that isn't letter
                  |               #    OR
                  \b[^{letter}\W] # does not start with a non-word character or letter
                  \w*             # zero or more additional word characters
                  {letter}\b      # ends with letter
                  '''.format(letter=t)
    return re.findall(pattern,s)

s = "The TART program runs on Tuesdays and Thursdays, but it does not start until next week."
print(getWords(s,'t'))

Output:

['The', 'Tuesdays', 'Thursdays', 'but', 'it', 'not', 'start', 'next']

Answer 2

Doing this is much easy with the startswith() and endswith() method.

def getWords(s, letter):
    return ([word for word in mystring.split() if (word.lower().startswith('t') or 
                word.lower().endswith('t')) and not 
                    (word.lower().startswith('t') and word.lower().endswith('t'))])

mystring = "The TART program runs on Tuesdays and Thursdays, but it does not start until next week."
print(getWords(mystring, 't'))

Output

['The', 'Tuesdays', 'Thursdays,', 'but', 'it', 'not', 'start', 'next']

---

Update (using regular expression)

import re
result1 = re.findall(r'\b[t]\w+|\w+[t]\b', mystring, re.I)
result2 = re.findall(r'\b[t]\w+[t]\b', mystring, re.I)
print([x for x in result1 if x not in result2])

Explanation

Regular expression \\b[t]\\w+ and \\w+[t]\\b finds words that start and ends with letter t and \\b[t]\\w+[t]\\b finds words that both starts and ends with letter t .

After generating two lists of words, just take the intersection of those two lists.

Answer 3

Why are you using regex for this? Just check the first and last character.

def getWords(s, letter):
    words = s.split()
    return [a for a,b in ((word, set(word.lower()[::len(word)-1])) for word in words) if letter in b and len(b)==2]

Answer 4

It you want the regex for this, then use:

regex = r'\b(#\w*[^#\W]|[^#\W]\w*#)\b'.replace('#', letter)

The replace is done to avoid the repeated verbose +letter+ .

So the code looks like this then:

import re

def getWords(sentence, letter):
    regex = r'\b(#\w*[^#\W]|[^#\W]\w*#)\b'.replace('#', letter)
    return re.findall(regex, sentence, re.I)

s = "The TART program runs on Tuesdays and Thursdays, but it does not start until next week."
result = getWords(s, "t")
print(result)

Output:

['The', 'Tuesdays', 'Thursdays', 'but', 'it', 'not', 'start', 'next']

Explanation

I have used # as a placeholder for the actual letter, and that will get replaced in the regular expression before it is actually used.

• \\b : word break
• \\w* : 0 or more letters (or underscores)
• [^#\\W] : a letter that is not # (the given letter)
• | : logical OR. The left side matches words that start with the letter, but don't end with it, and the right side matches the opposite case.

Answer 5

You can try the builtin startswith and endswith functions.

>>> string = "The TART program runs on Tuesdays and Thursdays, but it does not start until next week."
>>> [i for i in string.split() if i.lower().startswith('t') or i.lower().endswith('t')]
['The', 'TART', 'Tuesdays', 'Thursdays,', 'but', 'it', 'not', 'start', 'next']