This quantifier cannot appear after an opening parenthesis, because there is nothing to be made optional at the start of a group. The final question mark is the quantifier that makes the previous token optional. The question mark after the opening parenthesis is unrelated to the question mark at the end of the regex. The question mark and the colon after the opening parenthesis are the syntax that creates a non-capturing group. If you do not need the group to capture its match, you can optimize this regular expression into Set (?: Value ) ?. In the second case, the first capturing group matches Value.

In the first case, the first (and only) capturing group remains empty.

The regex Set ( Value ) ? matches Set or SetValue. It stores the part of the string matched by the part of the regular expression inside the parentheses. Parentheses Create Numbered Capturing Groupsīesides grouping part of a regular expression together, parentheses also create a numbered capturing group. Square brackets define a character class, and curly braces are used by a quantifier with specific limits. Only parentheses can be used for grouping. This allows you to apply a quantifier to the entire group or to restrict alternation to part of the regex. Use Parentheses for Grouping and Capturingīy placing part of a regular expression inside round brackets or parentheses, you can group that part of the regular expression together.

Curly brackets regex