RegExpEval is a tool to verify your regular expression

In this example I search for a valid date. Matches are marked in the sample text. If there is more then one match, the matches are marked by alternating colors. In the most upper line you type in your regular expression. The line below is for copying the regular expression with escaped '\', the number of escape characters you can choose at right side of this line. The upper box on the right side contained all matches, for each match you see the start and end position in sample text.

Select a match: When you choose a match, just this match is marked in the sample text. When the match contains capture groups, you will find them in the box below the matches. There are three capture groups, one for the day, one for the month and one for the year. The yellow color in the marked sample text indicates, that these characters are not captured by a capture group.

Select a capture group: You can choose a capture group, then just this part is marked in the sample text. In the example below, it is the year of the matched date. The numbers of the capture group indicates start- and end-position in sample text. When [All] is pressed, the whole match is marked again in sample text.