Is there something in regex to match (conditionally) only if it exists ?
e.g.
the string maybe
question_1
or only
question
In case of the former, it should match the integer in the end, but as in the case of the latter, it should leave it.
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2What does "leave it" mean? Do you want to match either string regardless of whether or not there is an underscore and a number at the end?– Andrew HareCommented Oct 8, 2011 at 12:26
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1This question denotes a null knowledge of Regexes. Not even zero. Null. SO isn't a tutorial. When you post here you should know at least the basics of the argument you are asking about.– xanatosCommented Oct 8, 2011 at 12:33
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Are you trying to say, If there is an underscore, a number must be followed, otherwise there should be no underscore at the end ?– AnwarCommented Jun 5, 2016 at 7:59
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5 Answers
The ? is the 0-1 quantifier in Regexes. \d? means 0 or 1 digit. The * is the 0-infinite quantifier. \d* means 0 or more digits. Is it what you want?
(additionally the + is the 1 or more quantifier, and not quantifier means exactly 1)
To elaborate on what you asked, I would say
question(_\d+)?
question followed by an optional (_ AND 1 or more digits)
Where the brackets are only to group the sub expression (they are "mathematical" brackets)
Don't quite understand the question. Do you just want to extract the number?
question_(\d+)
If you want to extract the number
question(?:_?)(\d)?
in perl you could do something like:
my $string = 'question_1';
my $question_number = $string =~ /question_(\d+)/i;
now $question_number will hold the int if it matches and will be undef if it doesn't
answered Oct 8, 2011 at 12:25
If the underscore and the number are optional try something like this:
question(?:_\d)?
answered Oct 8, 2011 at 12:27
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He tried to say, if there is an underscore, then a number should be present. otherwise, only the "question" without no ending _– AnwarCommented Jun 5, 2016 at 7:58