I want to match any string that starts with . and word and then optionally any character after a space.

r"^\.(\w )(?:\s (. )\b)?"

eg:

should match

.just one two
.just
.blah one@nine
.blah
.jargon blah

should not match

.jargon

I want this second group mandatory if first group is jargon

CodePudding user response：

One approach would be to phrase your requirement using an alternation:

^\.(?:(?!jargon\b)\w (?: \S )*|jargon(?: \S ) )$

This pattern says to match:

^                    from the start of the input
\.                   match dot
(?:
    (?!jargon\b)\w   match a first term which is NOT "jargon"
    (?: \S )*        then match optional following terms zero or more times
    |                OR
    jargon           match "jargon" as the first term
    (?: \S )         then match mandatory one or more terms
)
$                    end of the input

Here is a sample Python script:

inp = [".just one two", ".just", ".blah one@nine", ".blah", ".jargon blah", "jargon"]
matches = [x for x in inp if re.search(r'^\.(?:(?!jargon\b)\w (?: \S )*|jargon(?: \S ) )$', x)]
print(matches)  # ['.just one two', '.just', '.blah one@nine', '.blah', '.jargon blah']

CodePudding user response：

You could attempt to match the following regular expression:

^\.(?!jargon$)\w (?= .|$).*

Demo

If successful, this will match the entire string. If one simply wants to know if the string conforms to the requirements .* can be dropped.

(?!jargon$) is a negative lookahead that asserts that the period is not immediately followed by 'jargon' at the end of the string.

(?= .|$) is a positive lookahead that asserts that the string of word characters is followed by a space followed by any character or they terminate the string.