Regexper is the framework from which Regexplained @ForbesLindesay branches and therefore deserves a mention here.
It is worth noting that (at the time of this writing) the two syntax diagram generators were slightly different, but possibly significantly in terms of their results. Where Regexper seems more concise, Regexplained is, in my opinion, so verbose that it is at least redundant, and in the worst case misleading. No disrespect is assumed.
Example: comparing the next regular expression that matches repeated or ambiguous values from 1 to 7, separated by commas.
([1-7]{1}),(?:[1-7]{1},)*(?:\1|[^1-7,]|(?:[1-7]{2,}))
Syntax diagram via regexper.com
Syntax diagram via regexplained.co.uk
In the latter case, note how the path for the expression [1-7]{1} loops with the label "1 time".
In the best case, this is redundant, but in the worst case it can mislead the reader, believing that the expression must be matched twice, because the expression must be read once before the reader is technically allowed to follow the looped path (see: "Quantifiers "). "in the section" Reading railway schemes " ).
This implies that you must execute the loop “1 time” after reading the expression.
This notation is obvious in the expressions (?:[1-7]{1},)* and (?:[1-7]{2,}) .
At first glance, the Regexplained syntax seems correct, especially for an expression such as (?:[1-7]{2,}) , where it initially makes sense to see the output "2+ times" for {2,} . In fact, I preferred this entry for a while.
But technically, the expression is read once before the loop path. Thus, the loop should only occur “1+ times”, as correctly indicated in the Regexper syntax diagram.
For this reason, I prefer and recommend Regexper.