"Looking different directions" by Paul Kline at (https://www.flickr.com/photos/paulelijah/6717953239/)

“Looking different directions” by Paul Kline.

Problem: Match strings that contains a single quotation mark ('), but not multiple ones together.

Solution:

(?<!')'(?!')

This is a regex for a single quotation mark with a (?<!') in the left and a (?!’) in the right. The (?<!') is a ?< look behind if not ! a single quotation mark '. The (?!') is a look ahead ? if not ! a single quotation mark '.

Java code:

import java.util.regex.Pattern;

public class RegexProblem {
  public static void main(String args[]) {
    Pattern single_quote = Pattern.compile("(?<!')'(?!')");
    String[] phrases = {
      "",
      "'",
      "a'a",
      "aaa",
      "aa'aa",
      "aa''aa",
      "aa'''aaa",
      "aaa''''aaa"
    };
    for(String phrase: phrases){
      System.out.println(String.format("For %s is %s.", phrase,
            single_quote.matcher(phrase).find()));
    }
  }
}

The output is:

For  is false.
For ' is true.
For a'a is true.
For aaa is false.
For aa'aa is true.
For aa''aa is false.
For aa'''aaa is false.
For aaa''''aaa is false.