Regular expressions. Match specific word between two words

Question

I use C#. I have a string:

wordA wordB wordC wordB wordD

I need to match all occurrences of wordB between wordA and wordD. I use lookahead and lookbehind to match everything between wordA and worD like this:

(?<=wordA)(.*?)(?=wordD)

But something like

(?<=wordA)(wordB)(?=wordD) 

matches nothing. What would be the best way to match all occurrences of wordB between wordA and wordD?

Answer 1

Put the .*? into the lookarounds:

(?<=wordA.*?)wordB(?=.*?wordD)

See the regex demo

Now, the pattern means:

• (?<=wordA.*?) - (a positive lookbehind) requires the presence of wordA followed with any 0+ chars (as few as possible) immediately before...
• wordB - word B
• (?=.*?wordD) - (a positive lookahead) requires the presence of any 0+ chars (as few as possible) followed with a wordD after them (so, it can be right after wordB or after some chars).

If you need to account for multiline input, compile the regex with RegexOptions.Singleline flag so that . could match a newline symbol (or prepend the pattern with (?s) inline modifier option - (?s)(?<=wordA.*?)wordB(?=.*?wordD) ).

If the "words" consist of letters/digits/underscores, and you need to match them as whole words, do not forget to wrap the wordA , wordB and wordD with \\b s (word boundaries).

Always test your regexes in the target environment:

var s = "wordA wordB wordC wordB \n wordD";
var pattern = @"(?<=wordA.*?)wordB(?=.*?wordD)";
var result = Regex.Replace(s, pattern, "<<<$&>>>", RegexOptions.Singleline);
Console.WriteLine(result);
// => wordA <<<wordB>>> wordC <<<wordB>>> 
//    wordD

See C# demo .