what's wrong with this Regex? (Look-Ahead and Look-Behind)

Question

I try to extract the media description of a SDP package.

I have a line like

a=rtpmap:113 H264/90000

(after that there is always a carriage return)

I want everything after a=rtpmap:[number][whitespace] till the carriage return, without the return itself and constructed the following Regex, which doesn't work:

(?<=(a=rtpmap:[0-9]+\s)).*(?=\r)

If I do like

(?<=(a=rtpmap:[0-9]{3}\s)).*

I get at least a result, but the number after rtpmap isn't always 3 characters long. Why isn't the + working here?

I want only H264/90000; not the carriage return afterwards, not the a=rtpmap:[number][whitespace] before and no result, if there is no a=rtpmap:[number][whitespace] — Michael Niemand, Apr 04 '12 at 11:12

score 4 · Accepted Answer · answered Apr 04 '12 at 11:12

Many regex engines do not support arbitrary- or variable-length lookbehind. .NET does but it's nearly alone in that regard.

In your case you should be better off using something like the following:

a=rtpmap:\d+ ([^\r]+)

and using the first capturing group.

score 0 · Answer 2 · answered Apr 04 '12 at 11:13

0

Use this instead

(?<=(a=rtpmap:(\d+)\s)).*

answered Apr 04 '12 at 11:13

Bikee

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what's wrong with this Regex? (Look-Ahead and Look-Behind)

2 Answers2