Regular expressions are used to find matches in texts. The following is a real application of Regex in C# and Java. 

CSV are files that all the data is separated by a comma. E.g:
name,line1,line2,city,zip code,country
You cand easily use String.Split() in C# to get all the values. But, there are cases when the data can contain comma. E.g:
"Mr. John Doe, Jr.",7926 Glenbrook Dr., 14623
In this case a regular expression (regex) could be use to determine if the comma is inside a quote or not.
 
C# Example:
 
    public string[] parseCSV(string line)
    {
        List<string> datalist = new List<string>();

    /*
     * Define a regular expression for csv.
     * This Pattern will match on either quoted text or text between commas, including
     * whitespace, and accounting for beginning and end of line.
     */

    Regex rx = new Regex(&quot;\&quot;([^\&quot;]*)\&quot;|(?&lt;=,|^)([^,]*)(?:,|$)&quot;,
      RegexOptions.Compiled | RegexOptions.IgnoreCase);

    // Find matches.
    MatchCollection matches = rx.Matches(line);

    // Report the number of matches found.
    Console.WriteLine(&quot;{0} matches found.&quot;, matches.Count);

    // Report on each match.
    foreach (Match match in matches)
    {
        if (match.Groups[1].Value.Length &gt; 0)
            datalist.Add(match.Groups[1].Value); // match csv values inside commas
        else
            datalist.Add(match.Groups[2].Value); // match csv values outside commas
    }
    return datalist.ToArray();
}</pre>
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    Java Example:</div>
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    <pre>
public String[] parse(String csvLine) {
    Pattern csvPattern = Pattern.compile(&quot;\&quot;([^\&quot;]*)\&quot;|(?&lt;=,|^)([^,]*)(?:,|$)&quot;);
    matcher = csvPattern.matcher(csvLine);
    allMatches.clear();
    String match;

    while (matcher.find()) {
            match = matcher.group(1);

            if (match!=null) {
                    allMatches.add(match);
            }
            else {
                    allMatches.add(matcher.group(2));
            }
    }

    size = allMatches.size();
    if (size &gt; 0) {
            return allMatches.toArray(new String[size]);
    }
    else {
            return new String[0];
    }
} </pre>
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