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<\/b>Doesn’t matter even if you are a ninja programmer or a novice, at a point in life, you got to master regular expression. Therefore, the sooner the better.<\/p>\n
So, let\u2019s get started.<\/p>\n
Many people refer to call regular expression as Regex<\/b><\/span>, Regexp<\/b><\/span> or simple Regular expression<\/b><\/span>. All means the same. Regex is a pattern that consists of rules used to match a certain set of strings. For example, we can write the names (three bold words above) in a regex form like this<\/p>\n <\/p>\n \/^<\/b>Reg(exp?|ular expression)<\/b><\/span>$<\/span>\/<\/b><\/p>\n <\/p>\n A more abstract definition would be, “regex is a way to describe a set strings”. Now the more pressing question is why regex are useful or should you use regex at all?<\/p>\n <\/p>\n <\/p>\n We can check if a string matches a certain format, like in HTML5, we can use it in form validation. We can use it to extract certain parts of the string or replace, depends on your need. Regex capabilities are built-in in many programming languages, IDEs, command line tools like grep, sed, etc.; And also with databases. Regex are used in search engine.<\/p>\n <\/p>\n One limitation of regex would be that, they are not fully portable, means regex written for ruby might not work with PHP. This is due to the different regex engines (PCRE, Jregex, XregExp).<\/p>\n <\/p>\n Now let’s dive into regex basic:<\/p>\n <\/p>\n Common Matching Symbols<\/b><\/span><\/p>\n Dot (.)<\/strong><\/span><\/p>\n It matches anything except newline.<\/p>\n <\/p>\n {n}, {n, m}, {n, }<\/b><\/span><\/p>\n To match same consecutive symbols over and over again, we can use curly-braces and to define length of character(s). They are also called quantifiers.<\/p>\n <\/p>\n Above regex will match c<\/span>, 10 consecutive a‘s<\/span><\/span> and a t<\/span>.<\/p>\n {n} = matches exactly \u2018n\u2019 times.<\/p>\n {n, } = matches at least \u2018n\u2019 times but can match farther times, latter is known as greediness.<\/p>\n {n, m} = matches at least \u2018n\u2019 times, but no more than \u2018m\u2019 times, \u2018m\u2019 is just the upper bound here.<\/p>\n <\/p>\n There are certain shortcut for defining quantifiers.<\/p>\n <\/p>\n * = {0, }<\/p>\n *<\/span> matches 0 or more number of time, it also matches empty strings if there are zero matches.<\/p>\n + = {1, }<\/p>\n +<\/span> matches 1 or more number of time, it does not matches empty strings.<\/p>\n ? = {0, 1}<\/p>\n ?<\/span> matches either 0 or 1 number of time, It also matches empty strings if there are zero match.<\/p>\n <\/p>\n Question<\/b><\/span>: Match the highlighted text using quantifiers?<\/p>\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 <\/b>\u00a0 \u00a0 <title><\/span>aaaabbbbbccccc<<\/span>\/title><\/span><\/p>\n <\/p>\n Solution<\/b>:<\/span> A poor regex would be this<\/span> <.+><\/span> because it won\u2019t give you the correct answer. Why?<\/span><\/span><\/p>\n Let\u2019s analyse it.<\/span><\/p>\n S<\/b>tep 1<\/b>:<\/span><\/p>\n String: <<\/span>title>aaaabbbbbccccc<\/title><\/p>\n Regex: <<\/span>.+><\/p>\n The regex starts with <<\/span> therefore, it will just match the angle bracket.<\/p>\n <\/p>\n Step 2<\/b>:<\/span><\/p>\n String: <title>aaaabbbbbccccc<\/title><\/span><\/p>\n Regex: <<\/span>.+<\/span>><\/p>\n The dot (.<\/span>) will match only one character and using it with +<\/span> will make it to match at least one character or more, as quantifiers are greedy, therefore it will match the whole\u00a0string.<\/p>\n <\/p>\n Step 3<\/b>:<\/span><\/p>\n String: <title>aaaabbbbbccccc<\/title><\/span><\/span><\/p>\n Regex: <.+<\/span>><\/span><\/p>\n Now the regex engine will try to find the closing angle bracket, for this it will do backtracking from the end until it reaches the first angle bracket.<\/p>\n Result: <title>aaaabbbbbccccc<\/title><\/span><\/p>\n <\/p>\n Technically regex did what it suppose to do. But this isn\u2019t what we wanted.<\/p>\n So the main culprit here is the greediness. To stop greediness use ?<\/span> in regex.<\/p>\n <\/p>\n Regex: <.+?><\/span><\/p>\n String: <title><\/span>aaaabbbbbccccc<\/title><\/span><\/p>\n now it match as little as possible.<\/p>\n <\/p>\n Brackets [ ]<\/span><\/b><\/span><\/p>\n [ ]<\/span> <\/span>are used to set alternation between characters or ranges or both.<\/p>\n <\/p>\n Question<\/b>: Match using <\/span><\/span>[ ]<\/span><\/p>\n Solution<\/b>: <\/span>[cmfj]<\/span>an<\/p>\n <\/p>\n To avoid writing all characters inside [ ]<\/span> you can write ranges like [a-z]<\/span> will match all small English characters.<\/p>\n You can also combine different ranges like this [a-zA-Z0-9]<\/span>, this will match all capital, small alphabets, and numbers. And using a quantifier with [ ]<\/span> can match multiple characters.<\/p>\n <\/p>\n Match<\/span><\/strong>: aabbccddeeaabbzztt<\/p>\n Regex<\/strong><\/span>: [a-z]+<\/span><\/p>\n <\/p>\n Instead of writing long ranges of characters, numbers inside [ ]<\/span> you can use character classes.<\/p>\n <\/p>\n [\\w<\/span>] = [a-zA-Z0-9_]<\/p>\n It contain plain old English characters, numbers and an underscore, it will not match \u2126<\/strong> or \u03d5 <\/strong>kind of\u00a0symbols.<\/p>\n <\/p>\n [\\d<\/span>] = [0-9]<\/p>\n It contain numbers from 0 to 9.<\/p>\n <\/p>\n [\\s<\/span>]\u00a0\u2248 [\\n\\t\\r ]<\/p>\n This class contain a tab, newline, vertical tab, form feed, carriage return and space.<\/p>\n <\/p>\n Question<\/b>: <\/span>Match hex-color<\/p>\n Solution<\/b>:<\/span> Hex colors are about of 3 and 6 characters long which contain alphabets from ‘a\u2019 to \u2018f\u2019 and \u2018A\u2019 to \u2018F\u2019 and numbers, which means #f00 is valid hex color code and #K00, #f002 are invalid. Therefore, we need to filter out those hex colors which have wrong alphabets and are of the wrong length.<\/span><\/p>\n <\/p>\n\n
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\n Match<\/span><\/td>\n can<\/span><\/td>\n<\/tr>\n \n Match<\/span><\/td>\n man<\/span><\/td>\n<\/tr>\n \n Match<\/span><\/td>\n fan<\/span><\/td>\n<\/tr>\n \n Match<\/span><\/td>\n jan<\/span><\/td>\n<\/tr>\n \n Skip<\/span><\/td>\n ran<\/span><\/td>\n<\/tr>\n \n Skip<\/span><\/td>\n tan<\/span><\/td>\n<\/tr>\n \n Skip<\/span><\/td>\n pan<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/dd>\n<\/dl>\n \n
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\n Match<\/span><\/td>\n #abc<\/span><\/td>\n<\/tr>\n \n Match<\/span><\/td>\n #f55<\/span><\/td>\n<\/tr>\n \n Match<\/span><\/td>\n #XTQ67A<\/span><\/td>\n<\/tr>\n \n Match<\/span><\/td>\n #C0FFEE<\/span><\/td>\n<\/tr>\n \n Skip<\/span><\/td>\n #!AB<\/span><\/td>\n<\/tr>\n \n Skip<\/span><\/td>\n #!f004<\/span><\/td>\n<\/tr>\n \n Skip<\/span><\/td>\n #f5Y<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/dd>\n<\/dl>\n