I have to parse a string using regular expressions in which at least one group in a set of two is required. I cannot figure out how to write this case.
To illustrate the problem we can think parsing this case:
String: aredhouse theball bluegreencar the
Match: ✓ ✓ ✓ ✗
Each item is composed by an article, a colour and an object defined by groups in the following expression (?P<article>the|a)?(?P<colour>(red|green|blue|yellow)*)(?P<object>car|ball|house)?\s*
An item can have an 'article' but must have a 'colour' or/and an 'object'.
Is there a way of making 'article' optional but require at least one 'colour' or 'object' using regular expressions?
Here is the coded Go version of this example, however I guess this is generic regexp question that applies to any language.
This is working with your testcases.
/
(?P<article>the|a)? # optional article
(?: # non-capture group, mandatory
(?P<colour>(?:red|green|blue|yellow)+) # 1 or more colors
(?P<object>car|ball|house) # followed by 1 object
| # OR
(?P<colour>(?:red|green|blue|yellow)+) # 1 or more colors
| # OR
(?P<object>car|ball|house) # 1 object
) # end group
/x
It can be reduced to:
/
(?P<article>the|a)? # optional article
(?: # non-capture group, mandatory
(?P<colour>(?:red|green|blue|yellow)+) # 1 or more colors
(?P<object>car|ball|house)? # followed by optional object
| # OR
(?P<object>car|ball|house) # 1 object
) # end group
/x
In regex, there's a few special signs that indicate the expected number of matches for a character or a group:
* - zero or more
+ - one or more
? - zero or one
These applied, your regex looks like this:
(?P<article>(the|a)?)(?P<colour>(red|green|blue|yellow)+)(?P<object>(car|ball|house)+)\s*