Changes

Tutorial9: Regular Expressions

456 bytes added, 11:51, 7 March 2021

no edit summary

# Issue the following Linux pipeline command to display whole positive or negative integers: grep "^[+-][0-9][0-9]*$" numbers1.dat | tee signed.txt What did you notice? Positive and negative numbers display, not 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 only whole numbers (with or without a positive or negative sign): 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/~~scripts~~myscripts/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.

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

# 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 typing.

# 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/~~scripts~~myscripts/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 should notice valid and more invalid numbers contained in this file. When finished, exit the more command.

# Reissue the previous pipeline command including a space in 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 or more occurrences of the word "'''the '''" or 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/~~scripts~~myscripts/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.

: 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.

# 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~~, more and less commands~~ ''' 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''' 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 '''more''' and '''less''' commands can search for the string "'''uli101'''".]]

# Issue the following Linux command to view the contents of the '''large-file.txt''': more 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" on the second line at the top.

#Search for the next occurrence of the pattern '''uli101''' by '''re-typing''' the following regular expression and pressing ENTER: /uli101 you should now see the '''second occurrence''' of this pattern near the top.

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

# Try the same search techniques with the '''less''' command. Does it work the same for the ''less'' command as it did for the ''more'' 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 move to the '''first occurrence''' of the pattern: '''uli101'''. Let's search for the '''uli101''' pattern, but 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

# Navigate throughout the text file to confirm that ALL occurrences of uli101 have been replaced with ULI101.

Msaul

Administrators

13,420

edits

Changes

Tutorial9: Regular Expressions

Navigation menu

Personal tools

Namespaces

Variants

Views

More

Search

Navigation

get involved with CDOT

courses

course projects

links

Tools