Regex Lookahead and Lookbehinds: followed by this or that

Question

I'm trying to write a regular expression that checks ahead to make sure there is either a white space character OR an opening parentheses after the words I'm searching for. Also, I want it to look back and make sure it is preceded by either a non-Word (\W) or nothing at all (i.e. it is the beginning of the statement).

So far I have,

"(\\W?)(" + words.toString() + ")(\\s | \\()"

However, this also matches the stuff at either ends - I want this pattern to match ONLY the word itself - not the stuff around it.

I'm using Java flavor Regex.

Answer 1

As you tagged your question yourself, you need lookarounds:

String regex = "(?<=\\W|^)(" + Pattern.quote(words.toString()) + ")(?= |[(])"

• (?<=X) means "preceded by X"
• (?<!=X) means "not preceded by X"
• (?=X) means "followed by X"
• (?!=X) means "not followed by X"

Answer 2

What about the word itself: will it always start with a word character (i.e., one that matches \w)? If so, you can use a word boundary for the leading condition.

"\\b" + theWord + "(?=[\\s(])"

Otherwise, you can use a negative lookbehind:

"(?<!\\w)" + theWord + "(?=[\\s(])"

I'm assuming the word is either quoted like so:

String theWord = Pattern.quote(words.toString());

...or doesn't need to be.

Answer 3

If you don't want a group to be captured by the matching, you can use the special construct (?:X)

So, in your case:

"(?:\\W?)(" + words.toString() + ")(?:\\s | \\()"

You will only have two groups then, group(0) for the whole string and group(1) for the word you are looking for.