Given two regexs expr1 and expr1, can we write a lookbehind (?<=expr1)expr2 equivalently in terms of the if-then-else construct?
For example, is the lookbehind (?<=expr1)expr2 equivalent to (expr1)(?(-1)expr2|expr3), where expr3 is some regex which is never possible to match?
In other words, are the two regex's (?<=expr1)expr2 and(expr1)(?(-1)expr2|expr3) describe the same?
If yes, how do you choose expr3, so that it is never possible to match?
Thanks?
Do the two regex's (?<=expr1)expr2 and (expr1)(?(-1)expr2|expr3) describe the same?
No. They match different strings.
(?<=expr1)expr2 can only ever match one thing: expr2, and not just anywhere: the expr2 in expr1expr2.
In contrast, (expr1)(?(-1)expr2|expr3) can only ever match: expr1expr2.
Clearly, expr2 and expr1expr2 are different strings. The answer is no.
..but the other answer is Yes:
Can we write a lookbehind (?<=expr1)expr2 equivalently in terms of the if-then-else construct?
Yes. This uses in an-then-else (the else is implied): (?(?<=expr1)expr2), and it too matches expr1expr2. Of course, it too uses a lookbehind.
The "implied else" doesn't sound right to you? Add a |, as in (?(?<=expr1|)expr2)
What about expr3?
Your last regex (expr1)(?(-1)expr2|expr3) will never be able to match expr3. If you want it to have a chance, you would have to make the capture group optional: (expr1)?(?(-1)expr2|expr3)
