Using regex (in c#.net) is it possible to check the previous line of a string?
For instance, I need to select strings in which the previous line is not a series of asterisks (prev line:******)
views:
41answers:
2
+1
A:
You can use RegexOptions.MultiLine and then match something like the following:
(?<!^\*+$\r?\n?.*)foo
This matches "foo" only if the previous line doesn't consist of asterisks.
Joey
2009-11-13 11:00:43
+4
A:
(?m)^(?<!^\*+\r?\n).+
(?m) turns on multiline mode so ^ can match the beginning of a line. The lookbehind checks the previous line; if it succeeds (that is, it doesn't see a line of asterisks), .+ consumes the current line.
Alan Moore
2009-11-14 00:42:55
+1 for actually explaining the regex.
exhuma
2009-11-14 00:46:10
Old Macs use just `\r` for a newline. Perhaps the `\n` should be optional?
Jeremy Stein
2009-11-14 14:57:18
They can't both be optional; to match exactly one of any of the three kinds of separator you need something like `\r?\n|\r`. I suppose we should keep doing it that way, but I hardly ever saw `\r` in the wild even before they made the switch.
Alan Moore
2009-11-14 20:42:33
Oh, I didn't look at your regex closely enough. They could both be optional if they followed a `$`. So something like this: `$\r?\n?`. But you're right, that is a pretty obscure case.
Jeremy Stein
2009-11-15 19:41:44