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Tutorial9: Regular Expressions

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→‎INVESTIGATION 1: SIMPLE & COMPLEX REGULAR EXPRESSIONS

# Issue the following linux Linux command to '''download''' a text file to your '''home''' directory: wget <nowiki>https://ict.senecacollege.ca/~murray.saul/uli101/textfile1.txt</nowiki>

# View the contents of the '''textfile1.txt''' file using the '''more''' command see what data is contained in this file. Although there are several Linux commands that use regular expressions, we will be using the '''grep''' command for this investigation. [[Image:regexps-1.png|thumb|right|250px|Output of '''grep''' command matching simple regular expression "'''the'''" (only lowercase). Notice the pattern matches larger words like "'''their'''" or "'''them'''".]]

#Issue the following Linux command to match the pattern "'''the'''" within '''textfile1.txt''': grep "the" textfile1.txt Take a few moments to view the output and observe the matched patterns.

# Now, issue the grep Linux command with the -i option to ignore case sensitively: grep -i "the" textfile1.txt What do you notice is different when issuing this command? You will notice that the pattern "'''the'''" is matched including larger words like "'''them'''" and "'''their'''". You can issue the '''grep''' command with the -w option to only match the pattern as a '''word'''.

# Issue the following Linux command: grep -w -i "the" textfile1.txt You should now see only strings of text that match the word '''"the"''' (upper or lower case). Matching literal or simple regular expressions can be useful, but are '''limited''' in what pattens they can match. For example, you may want to search for a pattern located at the '''beginning''' or '''end''' of the string. There are other regular expression symbols that provide more '''precise''' search pattern matching. These special characters are known as '''complex''' and '''extended''' regular expressions symbols. In this investigation, we will focus on complex ''regular expressions'' and then discuss ''extended regular expressions'' in INVESTIGATION 2. <table align="right"><tr valign="top"><td>[[Image:regexps-2.png|thumb|right|280px|Anchoring regular expressions at the '''beginning''' of text.]]</td><td>[[Image:regexps-3.png|thumb|right|250px|Anchoring regular expressions at the '''ending''' of text.]]</td></tr></table>

# Issue the following Linux command: grep -w -i "^the" textfile1.txt The '''^''' symbol is referred to as an '''anchor'''. In this case, it only matches the word "'''the'''" (both upper or lowercase) at the beginning of the string.

# Issue the following Linux command: grep -w -i "the$" textfile1.txt The '''$''' symbol is used to anchor patterns at the end of the string.

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Tutorial9: Regular Expressions

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