Question

In Java, what regular expression would I use to match a string that has exactly one colon and makes sure that the colon appears before any whitespace?

For example, it should match these strings:

label: print "Enter input"
But: I still had the money.
ghjkdhfjkgjhalergfyujhrageyjdfghbg:
area:54

But not

label: print "Enter input:"
There was one more thing: I still had the money.
ghfdsjhgakjsdhfkjdsagfjkhadsjkhflgadsjklfglsd
area::54

Answer 1

If you use it with matches (which requires to match the entire string), you could use

[^\\s:]*:[^:]*

Which means: arbitrarily many non-whitespace, non-: characters, then a :, then more arbitrarily many non-: characters.

I’ve really only used two regex concepts: (negated) character classes and repetition.

If you want to require at least one character before or after :, replace the corresponding * with + (as jlordo pointed out in a comment).