lookahead

I have implemented a generator-based scanner in Python that tokenizes a string into tuples of the form (token type, token value) : for token in scan("a(b)"): print token would print ("literal", "a") ("l_paren", "(") ... The next task implies parsing the token stream and for that, I need be able to look one item ahead from the current one without moving the pointer ahead as well. The fact that iterators and generators do not provide the complete sequence of items at once but each item as needed makes lookaheads a bit trickier compared to lists, since the next item is not known unless __next__()

I need to write an XSD schema with a restriction on a field, to ensure that the value of the field does not contain the substring FILENAME at any location. For example, all of the following must be invalid: FILENAME ORIGINFILENAME FILENAMETEST 123FILENAME456 None of these values should be valid. In a regular expression language that supports negative lookahead, I could do this by writing /^((?!FILENAME).)*$ but the XSD pattern language does not support negative lookahead. How can I implement an XSD pattern restriction with the same effect as /^((?!FILENAME).)*$ ? I need to use pattern, because

There are some features in modern regex engines which allow you to match languages that couldn't be matched without that feature. For example the following regex using back references matches the language of all strings that consist of a word that repeats itself: (.+)\1 . This language is not regular and can't be matched by a regex that does not use back references. Does lookaround also affect which languages can be matched by a regular expression? I.e. are there any languages that can be matched using lookaround that couldn't be matched otherwise? If so, is this true for all flavors of

I am implementing the following problem in ruby. Here's the pattern that I want : 1234, 1324, 1432, 1423, 2341 and so on i.e. the digits in the four digit number should be between [1-4] and should also be non-repetitive. to make you understand in a simple manner I take a two digit pattern and the solution should be : 12, 21 i.e. the digits should be either 1 or 2 and should be non-repetitive. To make sure that they are non-repetitive I want to use $1 for the condition for my second digit but its not working. Please help me out and thanks in advance. polygenelubricants You can use this ( see on

In my home directory I have a folder drupal-6.14 that contains the Drupal platform. From this directory I use the following command: find drupal-6.14 -type f -iname '*' | grep -P 'drupal-6.14/(?!sites(?!/all|/default)).*' | xargs tar -czf drupal-6.14.tar.gz What this command does is gzips the folder drupal-6.14 , excluding all subfolders of drupal-6.14/sites/ except sites/all and sites/default , which it includes. My question is on the regular expression: grep -P 'drupal-6.14/(?!sites(?!/all|/default)).*' The expression works to exclude all the folders I want excluded, but I don't quite

I have the following regex in a C# program, and have difficulties understanding it: (?<=#)[^#]+(?=#) I'll break it down to what I think I understood: (?<=#) a group, matching a hash. what's `?<=`? [^#]+ one or more non-hashes (used to achieve non-greediness) (?=#) another group, matching a hash. what's the `?=`? So the problem I have is the ?<= and ?< part. From reading MSDN, ?<name> is used for naming groups, but in this case the angle bracket is never closed. I couldn't find ?= in the docs, and searching for it is really difficult, because search engines will mostly ignore those special