regex-lookarounds

Using one line of Perl code, what is the shortest way possible to print all the lines between two patterns not including the lines with the patterns? If this is file.txt: aaa START bbb ccc ddd END eee fff I want to print this: bbb ccc ddd I can get most of the way there using something like this: perl -ne 'print if (/^START/../^END/);' That includes the START and END lines, though. I can get the job done like this: perl -ne 'if (/^START/../^END/) { print unless (/^(START)|(END)/); };' file.txt But that seems redundant. What I'd really like to do is use lookbehind and lookahead assertions like

I'm doing some regular expression gymnastics. I set myself the task of trying to search for C# code where there is a usage of the as-operator not followed by a null-check within a reasonable amount of space. Now I don't want to parse the C# code. E.g. I want to capture code snippets such as var x1 = x as SimpleRes; var y1 = y as SimpleRes; if(x1.a == y1.a) however, not capture var x1 = x as SimpleRes; var y1 = y as SimpleRes; if(x1 == null) nor for that matter var x1 = x as SimpleRes; var y1 = y as SimpleRes; if(somethingunrelated == null) {...} if(x1.a == y1.a) Thus any random null-check will

I am having problems with the nested '+'/'-' lookahead/lookbehind in regex. Let's say that I want to change the '*' in a string with '%' and let's say that '\' escapes the next character. (Turning a regex to sql like command ^^). So the string '*test*' should be changed to '%test%' , '\\*test\\*' -> '\\%test\\%' , but '\*test\*' and '\\\*test\\\*' should stay the same. I tried: (?<!\\)(?=\\\\)*\* but this doesn't work (?<!\\)((?=\\\\)*\*) ... (?<!\\(?=\\\\)*)\* ... (?=(?<!\\)(?=\\\\)*)\* ... What is the correct regex that will match the '*'s in examples given above? What is the difference

I try to list all files in a directory that do not start with "Camera1", but end with ".png". For doing so, I am using a regular expression in list.files in R. To exclude "Camera1", I tried to use a negative lookahead, but it doesn't work. Where is my mistake? ;) list.files(pathToDirectory, pattern = "^(?!Camera1).*\\.png") I get the error: invalid 'pattern' regular expression Thanks in advance :) Looks like the default engine doesn't like lookarounds, so you need to use Perl. This works: dat <- c("Camera1.png", "Camera2.png", "hello.png", "boo") grep("^(?!Camera1).*\\.png", dat, value=T, perl

I am trying to write a regular expression to mask an email address. Example below. input: john.doe@example.en.com output: j*******@e*********.com I have tried the following but I just can't seem to get it working correctly. regex: (?<=.).(?=[^@]\*?@) output:j*******@example.en.com regex: (?<=.).(?=[^@]\*?)(?=[^\.]\*?\.) output:j******************.com Any help would be appreciated. demo Update with various masking email solutions foo@bar.com ⇒ f**@b**.com (current question) - s.replaceAll("(?<=.)[^@](?=[^@]*?@)|(?:(?<=@.)|(?!^)\\G(?=[^@]*$)).(?=.*\\.)", "*") (see the regex demo ) foo@bar.com ⇒

I've just learned about these two concepts in more detail. I've always been good with RegEx, and it seems I've never seen the need for these 2 zero width assertions. I'm pretty sure I'm wrong, but I do not see why these constructs are needed. Consider this example: Match a 'q' which is not followed by a 'u'. 2 strings will be the input: Iraq quit With negative lookahead, the regex looks like this: q(?!u) Without it, it looks like this: q[^u] For the given input, both of these regex give the same results (i.e. matching Iraq but not quit ) (tested with perl). The same idea applies to lookbehinds