How to find a word that starts with a specific character

1. If you want to match a single character, you don't need to put it in a character class, so s is the same than [s].

2. What you want to find is a word boundary. A word boundary \b is an anchor that matches on a change from a non word character (\W) to a word character (\w) or vice versa.

The solution is:

\bs\w+

this regex will match on a s with not a word character before (works also on the start of the string) and needs at least one word character after it. \w+ is matching all word characters it can find, so no need for a \b at the end.

See it here on Regexr

>>> import re
>>> text = "I was searching my source to make a big desk yesterday."
>>> re.findall(r'\bs\w+', text)
['searching', 'source']

For lowercase and uppercase s use: r'\b[sS]\w+'

Lambda style:

text = 'I was searching my source to make a big desk yesterday.'

list(filter(lambda word: word[0]=='s', text.split()))

Output:

['searching', 'source']

I know it is not a regex solution, but you can use startswith

>>> text="I was searching my source to make a big desk yesterday."
>>> [ t for t in text.split() if t.startswith('s') ]
['searching', 'source']