{"version":"https:\/\/jsonfeed.org\/version\/1","title":"mathspp.com feed","home_page_url":"https:\/\/mathspp.com\/blog\/tags\/grammars","feed_url":"https:\/\/mathspp.com\/blog\/tags\/grammars.json","description":"Stay up-to-date with the articles on mathematics and programming that get published to mathspp.com.","author":{"name":"Rodrigo Gir\u00e3o Serr\u00e3o"},"items":[{"title":"Finding all strings a regular expression can find","date_published":"2017-11-20T00:00:00+01:00","id":"https:\/\/mathspp.com\/blog\/regex-printer","url":"https:\/\/mathspp.com\/blog\/regex-printer","content_html":"

A regular expression<\/a>, without much rigor, is a very compact way of representing several different strings. Given a regular expression (regex), can I find out all the strings the regex can find?<\/p>\n\n

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Photo by Aaron Burden on Unsplash<\/figcaption><\/figure>

One common use of regular expressions is to look for strings that have a certain structure in a bigger string (say a text). As an example, the regular expression abc(d|e)<\/code> can be used to look for the strings \"abcd\" and \"abce\", where the character |<\/code> denotes we have to make a choice (see image above). Thus cat|dog<\/code> would match the strings \"cat\" and \"dog\". There are other special symbols that have meanings and purposes.<\/p>\n

\"two<\/p>\n

One very interesting question that arises is: given a regular expression, what are the strings matched by it? To answer that question I wrote a small Python program, that I called regexPrinter<\/code>, that prints all strings matched by a given regular expression! In order to manage that task, I chose a subset of the regex syntax that I wanted to be able to print and also decided that whenever a piece of a pattern was infinite, at a given point the program would just print \"...\" to denote that infinity. This way, for any regex given, the program always stops.<\/p>\n

The regexPrinter<\/code> supports:<\/p>\n