multiline regex pattern that match 3 digits

Question

Can anyone please help me with a regex pattern that will match and check the following scenarios? I'm trying to match text that's separated with return characters. Each line should only have 3 digits (\\d{3}) in it and up to 100 lines.

This is ok:

123
456
789

This is not ok:

123
123789
456

This is not ok (note the empty line in the middle and at end):

123

456

I would appreciate any suggestion and help. thanks.

Answer 1

So the entire file must look like this? Then try

new Regex(@"\A(?:\d{3}\r?\n)*\z")

Explanation:

\A     # Start of string
(?:    # Match the following (non-capturing) group:
 \d{3} #  - three digits
 \r?\n #  - one CRLF or LF (linebreak)
)*     # any number of times (0 or more)
\z     # until the very end of the string

If the file might not end with a newline (not sure from your description), you can use

new Regex(@"\A(?:\d{3}\r?$\n?)*\z", RegexOptions.Multiline)

This initially makes newlines optional ( \\r?\\n? ) but ensures that there is a line ending after every three-character bit by placing the end-of-line anchor $ between CR and LF, which is where (strangely) .NET thinks it should match.

Answer 2

One way to do this is simply to check each line against your initial regex (\\d{3}). Another way is to add \\r \\n to your regex and allow for repeats

"(\d{3}\r?\n)*"

in order to filter out case 3, you just need to add beginning and end to show you want to match the entire file; (^ specifies beginning, $ specifies end)

"^(\d{3}\r?\n)*$"