Re:Is there a regular expression to detect a valid regular e...
by Piskvorrr on Sunday September 08, 2019 @01:36PM
(#20911976)
In other words, "if we build a state machine which is different from a regex, it will perform differently than a regex would." That's trivially true, but is it interesting beyond restating "there is no Silver Bullet"?
Re:Is there a regular expression to detect a valid regular e...
by gpm on Sunday September 08, 2019 @01:50PM
(#20912069)
Technically we aren't discussing the regex itself here but the syntax for defining a regex.
You could use a different syntax for defining the same regex, and that syntax could have different properties. For instance you could use parantheses like racket where "(...)" and "[...]" have the same meaning, but "(...]" is not well formed syntax (if I remember my racket correctly). Using such a syntax to define regex a counter would no longer suffice to decide if a string is a regex.
Academically a regex is a string of of:
- character literals
- "epsilon" characters (meaning empty string)
- "+" characters (which means either what we see before the + or what we see after the +, in real-world regex these are usually represented as | instead of + as well as used implicitly in constructs like [a-zA-Z])
- "*" characters (same meaning as in real-world regex)
- And parentheses to allow controlling the order of operations