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Tutorial9: Regular Expressions

1,288 bytes added, 21:23, 30 October 2021

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→‎INVESTIGATION 2: EXTENDED REGULAR EXPRESSIONS

=INVESTIGATION 1: SIMPLE & COMPLEX REGULAR EXPRESSIONS=

'''ATTENTION''': Depending on your ULI101 instructor, you may be required to complete this tutorial for '''marks''' in this course. Please refer to your instructor's course notes and lecture notes regarding evaluation for this course. The due date for successfully completing this tutorial (i.e. '''tutorial 9''') is by '''Friday by midnight''' next week (i.e. '''Week 9'''). If your instructor has NOT assigned marks for completing this tutorial, you can perform it for '''practice'''.

In this investigation, you will learn how to use the '''grep''' command with '''simple and complex regular expressions''' to help search for ''patterns'' contained in text files.

# '''Login''' to your matrix account.

# Issue a Linux command to '''confirm''' you are located in your '''home''' directory. The '''wget''' command is used to download files from the Internet to your shell. This will be useful to download '''text files''' and '''data files''' that we will be using ~~for~~ in this tutorial.

# 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>

~~# Issue the '''ls''' Linux command to confirm that the text file was downloaded. ~~# View the contents of the '''textfile1.txt''' file using the '''more''' command ~~and quickly view the contents of~~ 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 ~~pattern~~patterns. # ~~Now, issue~~ 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 For the remainder of this investigation, we will focus on ~~complex~~ '''complex regular expressions''' and then ~~discuss~~ focus on ''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.

# Issue the following Linux command to anchor the word "'''the'''" '''simultaneously''' at the beginning and end of the string: grep -w -i "^the$" textfile1.txt What do you notice? Anchoring patterns at both the beginning and ending of strings can greatly assist for more '''precise''' search pattern matching. We will now be demonstrate the ~~power~~ '''effectiveness''' of ~~anchoring~~combining ~~combined~~ '''anchors''' with other complex regular expressions symbols. <table align="right"><tr valign="top"><td>[[Image:regexps-4.png|thumb|right|280px|Anchoring regular expressions using '''period''' symbols at the '''beginning''' of text.]]</td><td>[[Image:regexps-5.png|thumb|right|250px|Anchoring regular expressions using '''period''' symbols simultaneously at the '''beginning''' and '''ending''' of text.]]</td></tr></table># Issue the following Linux command to match strings that '''begin with 3 characters''': grep "^..." textfile1.txt What do you notice? Can lines that contain '''less than 3 characters''' be displayed? # Issue the following Linux command to match strings that '''begin and end with 3 characters''': grep "^...$" textfile1.txt What do you notice compared to the previous command? # Issue the following Linux command to match strings that '''begin with 3 digits''': grep "^[0-9][0-9][0-9]" textfile1.txt What did you notice? # Issue the following Linux command to match strings that '''end with 3 uppercase letters''': grep "[A-Z][A-Z][A-Z]$" textfile1.txt <table align="right"><tr valign="top"><td>[[Image:regexps-6.png|thumb|right|220px|Anchoring '''3 digits''' at the '''beginning''' and '''ending''' of text.]]</td><td>[[Image:regexps-7.png|thumb|right|250px|Anchoring '''3 alpha-numeric characters''' at the '''beginning''' and '''ending''' of text.]]</td></tr></table>~~Did any lines~~ What type of strings match this pattern? # Issue the following Linux command to match strings that '''consist of only 3 digits''': grep "^[0-9][0-9][0-9]$" textfile1.txt What did you notice? # Issue the following Linux command to match strings that '''consist of only 3 alphanumeric digits''': grep "^[a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9]$" textfile1.txt What did you notice? The "*" complex regular expression symbol is often confused with the "*" '''filename expansion''' symbol. In other words, it does NOT represent zero or more of '''any character''', but zero or more '''occurrences''' of the character that comes '''before''' the "*" symbol. # To demonstrate, issue the following Linux command to display '''zero or more occurrences ''' of the letter "'''x'''": grep "x*" textfile1.txt You will most likely notice most lines of the file is displayed. # Let's issue a Linux command to display strings that contain '''more than one occurrence ''' of the letter "'''x'''": grep "xx*" textfile1.txt Why did this work? because the pattern indicates one occurrence of the letter "x", followed by '''zero or MORE occurrences''' of the next letter "x". If you combine the complex regular expression symbols ".*" it will act like zero or more occurrences of any character (i.e. like "*" did in filename expansion).

# Issue the following Linux command to match strings begin and end with a number with nothing or anything inbetween: grep "^[0-9].*[0-9]$" textfile1.txt Using '''simultaneous anchors''' combined with the ".*" symbol(s) can help you to refine your search patterns of strings.

# Issue the following Linux command to display strings that begin with a capital letter, end with a number, and contains a capital X somewhere inbetween: grep "^[A-Z].*X.*[0-9]$" textfile1.txt Let's look at another series of examples involving searching for strings that only contain '''valid numbers'''. We will use '''pipeline commands''' to both display stdout to the screen and save to files for confirmation of running these pipeline commands when run a '''checking-script''' later in this investigation.

# Issue the following Linux pipeline command to display only '''signed''' integers: grep "^[+-][0-9][0-9]*$" numbers1.dat | tee signed.txt What did you notice? Positive and negative numbers display, not '''unsigned''' numbers. [[Image:regexps-8.png|thumb|right|300px|Simultaneous '''anchoring''' of regular expressions using '''character class''' and '''zero or more occurrences''' to display '''signed''' and '''unsigned''' integers.]]

# Issue the following Linux pipeline command to display '''signed''' or '''unsigned integers''': grep "^[+-]*[0-9][0-9]*$" numbers1.dat | tee all.txt Did this command work?

# Issue the following command to check that you created those hard links: ~~bash /home/murray.saul/myscripts~~~uli101/week9-check-1 If you encounter errors, then view the feedback to make corrections, and then re-run the checking script. If you receive a congratulation message that there are no errors, then proceed with this tutorial. You can also use the '''grep''' command using ''regular expression'' as a '''filter''' in pipeline commands.

# Issue the following Linux pipeline command: ls | grep "[0-9].*dat$" What did this pipeline display?

# Issue the following Linux pipeline command: ls | grep "[a-z].*txt$" What did this pipeline display?

: Although very useful, '''complex''' regular expressions do NOT entirely solve our problem of displaying ~~only~~ '''valid''' unsigned and signed numbers(not to mention displaying decimal numbers). In the next investigation, you will learn how to use '''extended''' regular expressions that will completely solve this issue.

: You can proceed to INVESTIGATION 2.

In this ~~section~~investigation, you will learn how to use '''extended regular expressions''' with the '''egrep''' command to further refine your search patterns.

# View the contents of the '''numbers2.dat''' file using the '''more''' command and quickly view the contents of this file. You should notice ''valid'' and ''invalid'' numbers contained in this file. When finished, exit the more command. [[Image:eregexps-1.png|thumb|right|300px|'''Weakness''' of '''complex''' regular expressions that do not '''limit''' the number of '''positive''' or '''negative''' signs.]]

# Issue the following Linux command to display '''signed''' or '''unsigned integers''': grep "^[+-]*[0-9][0-9]*$" numbers2.dat You should notice '''multiple''' '''+''' or '''-''' '''signs''' appear prior to some numbers. This occurs since you are searching or one or MORE occurrences of a + or - sign. Using '''extended regular expression''' symbols to specify '''minimum''' and '''maximum''' repetitions: '''{min,max}''' can solve that problem.

# Issue the following Linux command (using extended regular expression symbols) to display '''signed''' or '''unsigned integers''': grep "^[+-]{0.,1}[0-9]{1,}$" numbers2.dat '''NOTE: No output will be displayed! Why?''' This is due to the fact that the '''grep command was NOT issued correctly to use extended regular expression symbols'''. You would need to issue either '''grep -E''' (, or ~~more simply)~~ just issue the '''egrep''' command. The egrep command works with '''all''' regular expression symbols, and should be used in the future instead of the older grep command. We will use '''pipeline commands''' to both display stdout to the screen and save to files for confirmation of running these pipeline commands when run a '''checking-script''' later in this investigation.

# Issue the following Linux pipeline command using '''egrep''' instead of ''grep'': egrep "^[+-]{0,1}[0-9]{1,}$" numbers2.dat | tee better-number1.txt [[Image:eregexps-2.png|thumb|right|300px|Using '''extended''' regular expression symbols (such as '''repetition''') to refine matches of ''signed'' and ''unsigned'' integers.]]You should have noticed that the command worked correctly this time because you used the '''egrep''' command. '''NOTE:''' With extended regular expressions, the '''?''' symbol can be used to represent the '''{0,1}''' repetition symbols and the '''+''' symbol can be used to represent the '''{1,}''' repetition symbols

# Issue the following Linux pipeline command using the repetition shortcuts "+" and "?": egrep "^[+-]?[0-9]+$" numbers2.dat | tee better-number2.txt You should have seen the '''same results''', but ~~the extended regular expression required~~ less typingwas required.

# Issue the following Linux pipeline command to display '''signed''', '''unsigned''', '''whole''', and '''decimal''' numbers: egrep "^[+-]{0,1}[0-9]{1,}[.]{0,1}[0-9]*$" numbers2.dat | tee better-number3.txt Were all signed and unsigned intergers and decimal numbers displayed?

# Issue the follwoing command to check that you correctly issued those ''Linux pipeline commands'': ~~bash /home/murray.saul/myscripts~~~uli101/week9-check-2 If you encounter errors, then view the feedback to make corrections, and then re-run the checking script. If you receive a congratulation message that there are no errors, then proceed with this tutorial. You can also use extended regular expression symbols for '''grouping'''. For example, you can search for repetitions of GROUPS of characters (like a word) as opposed to just a single character or a GROUP of numbers as opposed to a single digit.

# Issue the following linux pipeline command to download another data file called '''words.dat''': wget <nowiki>https://ict.senecacollege.ca/~murray.saul/uli101/words.dat</nowiki>

# View the contents of the '''words.dat''' file using the '''more''' command and quickly view the contents of this file. ~~You~~ Within this file, you should notice ~~valid and more invalid numbers contained in this file~~some lines that contain repetitions of words. When finished, exit the more command.

# Issue the following linux pipeline command to display '''two or more occurrences''' of the word "the": egrep -i "(the){2,}" words.dat | tee word-search1.txt more '''NOTE: No output is displayed! Why?''' This is due to the fact that a space should be included at the end of the word "'''the'''". Usually words are separated by spaces; therefore, there were no matches since there were not occurrences of "thethe" as opposed to "'''the the'''" (i.e. no space after repetition of the pattern).

# Reissue the previous pipeline command ~~including~~ with the word the followed by a '''space in ''' within the brackets: egrep -i "(the ){2,}" words.dat | tee word-search2.txt [[Image:eregexps-3.png|thumb|right|330px|Using '''extended''' regular expression symbols (such as '''grouping''') to refine matches of repetition of '''words''' (as opposed to ''characters'').]]The "|" (or) symbol (same symbol as "pipe") can be used within the grouping symbols to allow matching of additional groups of characters. Again, it is important to follow the character groupings with the space character # Issue the following linux pipeline command to search for 2 '''two or more occurrences ''' of the word "'''the '''" or '''two or more occurrences''' of the word "'''and '''": egrep -i "(the |and ){2,}" words.dat | tee word-search3.txt # Issue the following Linux command to check that you correctly issued those ''Linux pipeline commands'' using the '''tee''' command to create those text files: ~~bash /home/murray.saul/myscripts~~~uli101/week9-check-3 If you encounter errors, then view the feedback to make corrections, and then re-run the checking script. If you receive a congratulation message that there are no errors, then proceed with this tutorial. Let's issue a Linux '''pipeline''' command using the '''egrep''' command as a ~~combination of~~ '''filter''' using both '''complex''' and '''extended''' regular expressions.

# Issue the following Linux pipeline command: ls | egrep "[a-z]{1,}.*[0-9]" What did this Linux pipeline command display?

: The '''grep''' and '''egrep''' Linux commands are NOT the only Linux commands that use regular expressions. In the next investigation, you will apply regular expressions to a number of Linux commands that you already learned in this course.

=INVESTIGATION 3: OTHER COMMANDS USING REGULAR EXPRESSIONS =

In this ~~section~~investigation, you will see commands other than '''grep''' or '''egrep''' that can use regular expressions.

# Make certain that you are located in your '''~/regexps''' directory on your ''Matrix'' account.

# Let's look at using regular expressions with the '''man''' command. Issue the following linux command : man ls [[Image:other-re-1.png|thumb|right|300px|Entering '''/sort''' in the '''man''' command can search for the string "'''sort'''".]]

# We want to search for an option that can sort the file listing. Type the following regular expression below and press '''ENTER''': /sort '''FYI:''' The '''grep''' and '''egrep''' Linux commands contain the regular expressions within quotes, but '''most''' other Linux commands specify regular expressions using '''forward slashes ''' (e.g. /regular expression   or   /regular expression/).

# Scroll throughout the man pages for the ls command to view matches for the pattern "'''sort'''" (You can press '''SPACE''' or key combination '''alt-b''' to move forward and backwards one screen respectively).

# Press the letter q to '''exit''' the ''man'' pages for '''ls'''. Let's use regular expressions with the '''~~more~~less''' command. # Issue the following Linux command to download another data file called '''large-file.txt''': wget <nowiki>https://ict.senecacollege.ca/~murray.saul/uli101/large-file.txt</nowiki>~~ # View the contents of the '''large-file.txt''' file using the '''more''' command and quickly view the contents of this file.~~ [[Image:other-re-2.png|thumb|right|300px|Entering '''/uli101''' in the '''less''' command can display all matches of "'''uli101'''" throughout the text file.]]

# Issue the following Linux command to view the contents of the '''large-file.txt''': less large-file.txt

#We want to search for a pattern '''uli101''' within this text file. Type the following regular expression and press ENTER: /uli101 You should see the pattern "uli101" throughout the text file.

# Press the letter q to exit the '''~~more~~less''' command.

# Try the same search techniques with the '''more''' command. Does it work the same for the ''less'' command? [[Image:other-re-3.png|thumb|right|300px|Entering '''/uli101''' in the '''vi''' command can search for the string "'''uli101'''".]]Let's learn how to perform a simple '''search and replace''' within the '''vi''' utility by using regular expressions.

# Issue the following Linux command to edit the '''large-file.txt''' file: vi large-file.txt Let's first perform a simple search within this text file.

# Press the '''ESC''' key to make certain you are in '''COMMAND''' mode.

# Type the following and press '''ENTER''': /uli101 You should notice the pattern "'''uli101'''" highlighted for ALL occurrences in this text file. Let's '''search''' for the '''uli101''' pattern, and '''replace''' it in capitals (i.e '''ULI101'''). In vi, to issue a command, you need to enter '''LAST LINE''' MODE then issue a command. Let's issue a command from '''LAST LINE''' MODE to search and replace '''uli101''' to '''ULI101'''. [[Image:other-re-4.png|thumb|right|500px|In l'''ast line''' MODE in the '''vi''' text editor, issuing a command using regular expressions to convert '''uli101''' to '''ULI101'''.]]

# Making certain that you are '''COMMAND''' MODE in vi, type the following and press '''ENTER''': :%s/uli101/ULI101/g '''NOTE:''' The letter '''g''' after the replace regular expression represents "'''global'''" and will replace ALL occurrences of uli101 in the text document (as opposed to replacing the first occurrence for every line). # Type the following (in uppercase letters) and press '''ENTER''': /ULI101 You should notice the pattern "'''ULI101'''" highlighted for ALL occurrences in this text file. # Navigate throughout the text file to confirm that ALL occurrences of '''uli101 ''' have been replaced with '''ULI101'''.

# Save changes to your vi editing session and exit by typing the following and pressing ENTER: :x

: After you complete the '''Review Questions''' section to get additional practice, then work on your '''online assignment 3'''.

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Tutorial9: Regular Expressions

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