How to write optional word in Regular Expression?

问题

I want to write a java Regular expression that recognises the following patterns. abc def the ghi and abc def ghi

I tried this:

abc def (the)? ghi

But, it is not recognizing the second pattern.Where am I going wrong?

回答1:

abc def (the )?ghi

           ^^

Remove the extra space

回答2:

Spaces are also valid characters in regex, so

abc def (the)? ghi
       ^      ^ --- spaces

can match only

abc def the ghi
       ^   ^---spaces

or when we remove the word

abc def  ghi
       ^^---spaces

You need something like abc def( the)? ghi to also make one of these spaces optional.

来源：https://stackoverflow.com/questions/32566087/how-to-write-optional-word-in-regular-expression