How does the () and [] come into play with regex?

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the r(ege)+x expression will match the letter r followed by one or more sets of the string of letters ege followed by the letter x

Possible matches would be “regex”, “regeegex”, “regeegeegex”, and so on.

The expression r[ege]+x will match the letter r followed by one or more of any of the letters in the square brackets, followed by the letter x.

Possible matches would be “rex”, “rgx”, “reeex”, “rgggx”, “regegeggggx”, etc. In the square bracketed expression, the second “e” is redundant, by the way.

You can test expressions using a site like http://regex101.com or similar.

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