But what if we wanted to just find all numerical strings that corresponded to years, i.e. The plus sign gets us all numerical strings, whether they consist of one number or 1,000 numbers. Try running the previous regular expression without the plus sign to see the difference. The result is a list of all strings that contain one or more consecutive numerical digits: 2014 In regular expression, the plus sign is a metacharacter used to indicate that the previous token should be matched one or more times: grep -oE '] ' excerpt.txt As with most things in computing, anytime you see physical repetition, there's usually a shorthand version. the token, ], is repeated 4 times in the above pattern. Let's refer to each instance of ] as being a token, i.e. Take a look at the four-digit pattern again: ]]]] ] - Space characters, including tabs and newlines ] - Numbers 0 to 9 ] - Lower-case letters Here's a few of the character classes that work with grep (and other Unix tools, such as tr): So, to find four numerical digits in a row: grep -oE "]]]]" excerpt.txt Instead, that pattern, ], is the regular expression syntax for match a numerical digit. In the following example that uses ], we are not searching for the literal pattern of ]. To reiterate the first example with 2014, we were searching for the literal pattern of 2014. The regular expression syntax for the character class of numerical digits is simply: \d. So instead of searching for the literal string, 2014, what if we could search for any sequence of four numerical digits? For example, to see if Obama mentioned any other year in his speech? But many times, we have no idea, especially when searching thousands or millions of text files and strings. Matching literal strings is nice when you know exactly what you want. Let's use the grep option, -o, to show only the exact match: grep -oE '2014' excerpt.txt The following invocation of grep and its extended regex option, -E, simply looks for 2014 in excerpt.txt – another way I like to phrase this is that grep is searching for the pattern that contains the literal string, "2014": grep -E '2014' excerpt.txtīy default, grep outputs every line that contains 2014: About a year ago, I promised that 2014 would be a breakthrough year for America. What sets a regular expression apart from your typical find-text function is its use of metacharacters to describe patterns to match, such as, any numerical digit or any non-alphabetical character or the end of a line.īut if we specify a regular expression without any metacharacters, then it simply acts as a straightforward text-finding function. We have cut our deficits by about two-thirds.Īnd after 13 long years, our war in Afghanistan has come to a responsible end, and more of our brave troops have come home. Thanks to the Affordable Care Act, about 10 million Americans have gained health insurance in the past year alone. Over a 58-month streak, our businesses have created 11.2 million new jobs.Īfter a decade of decline, American manufacturing is in its best stretch of job growth since the '90s.Īmerica is now the world’s number one producer of oil and gas, helping to save drivers about a buck-ten a gallon at the pump over this time last year. In 2014, unemployment fell faster than it has in three decades. That means that 2014 was the strongest year for job growth since the 1990s. In December, our businesses created 240,000 new jobs. I've reformatted it to put each sentence on its own line (this is due to a constraint of grep, which can only match patterns by line): Hi, everybody.Ībout a year ago, I promised that 2014 would be a breakthrough year for America.Īnd this week, we got more evidence to back that up. 10, 2015, weekly address, "Resurgence is Real". Eįor the purpose of this guide, we'll use the following excerpt (referred to as excerpt.txt) from President Obama's Jan. The easiest way to utilize regular expressions is through grep (check out the basic tutorial on grep if you haven't already) and by using its extended regular expression option, i.e. I like to think of regular expressions as simply, "finding text, on steroids." But the additional complexity allows us to greatly expand the way we search and filter text.Īt the most basic level, the use of regular expression is no different than doing a Ctrl-F to activate the "Find" function in your word processor. Their syntax represents a mini-language of their own (though not a complete programming language) that has to be memorized. Regular expressions are patterns for describing text that we want to find. A syntax for describing patterns of text, i.e.
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