In a regular expression, I need to know how to match one thing or another, or both (in order). But at least one of the things needs to be there.
For example, the following regular expression
While the following regular expression
will match all three of the strings above, but it will also match the empty string, which we do not want.
I need something that will match all three of the strings above, but not the empty string. Is there an easy way to do that?
Both Andrew's and Justin's below work for the simplified example I provided, but they don't (unless I'm mistaken) work for the actual use case that I was hoping to solve, so I should probably put that in now. Here's the actual regexp I'm using:
This will match
45 45.988 45,689 34,569,098,233 567,900.90 -9 -34 banana fries 0.56 points
but it WON'T match
and I need it to do this.
The fully general method, given regexes
Note the others have used the structure of your example to make a simplification. Specifically, they (implicitly) factorised it, to pull out the common
[0-9]+ factors on the left and right.
The working for this is:
[0-9]+, so pull that out:
?(i.e. use the equivalence
(|a|b) = (a|b)?):
(|a+)is the same as
a*, so, finally:
Yes, you can match all of these with such an expression:
Note, it also doesn't match the empty string (your last condition).
Sure. You want the optional quantifier,
The above is slightly awkward-looking, but I wanted to show you your exact pattern with some
?s thrown in. In this version,
(?=.) makes sure it doesn't accept an empty string, since I've made both clauses optional. A simpler version would be this:
This satisfies your requirements, including preventing an empty string.
Note that there are many ways to express this. Some are long and some are very terse, but they become more complex depending on what you're trying to allow/disallow.
If you want to match this inside a larger string, I recommend splitting on and testing the results with
/^\d*\.?\d+$/. Otherwise, you'll risk either matching stuff like
If you know for a fact that you won't get strings like the above, you can use word breaks instead of
^$ anchors, but it will get complicated because there's no word break between
That ought to do it. It will block stuff like
aaa123.456bbb, but it will allow
123.456. It will allow
aaa.123.456.bbb, but as I've said, you'll need two steps if you want to comprehensively handle that.
If you want to allow whitespace at the beginning, negative/positive marks, and words at the end, those are actually fairly strict rules. That's a good thing. You can just add them on to the simplest pattern above:
Allowing thousands groups complicates things greatly, and I suggest you take a look at the answer I linked to. Here's the resulting pattern:
\b ensures that the numeric part ends with a digit, and is followed by at least one whitespace.
Maybe this helps (to give you the general idea):
This pattern matches characters, digits, or digits following characters, but not characters following digits. The pattern matches aa, aa11, and 11, but not 11aa, aa11aa, or the empty string. Don't be puzzled by the ".^", which means "a character followd by line start", it is intended to prevent any match at all.
Be warned that this does not work with all flavors of regex, your version of regex must support
Nice answer by huon (and a bit of brain-twister to follow it along to the end). For anyone looking for a quick and simple answer to the title of this question, 'In a regular expression, match one thing or another, or both', it's worth mentioning that even (A|B|AB) can be simplified to:
Handy if B is a bit more complex.
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