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→INVESTIGATION 2: EXTENDED REGULAR EXPRESSIONS
# View the contents of the '''numbers2.dat''' file using the '''more''' command and quickly view the contents of this file.<br>You should notice valid and more invalid numbers contained in this file. When finished, exit the more command.<br><br>
# Issue the following linux pipeline command to display only whole numbers (with or without a positive or negative sign):<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[+-]*[0-9][0-9]*$" numbers2.dat | more</span><br><br>You should notice '''multiple''' '''+''' or '''-''' '''signs''' appear <u>prior</u> to some numbers.<br>This occurs since you are searching or one or MORE occurrences of a + or - sign.<br><br>Using '''extended regular expression''' symbols to specify '''minimum''' and '''maximum''' repetitions: '''{min,max}''' can solve that problem.<br><br>
# Issue the following linux pipeline command (using extended regular expression symbols) <br>to display only whole numbers (with or without a positive or negative sign):<br><span style="color:blue;font-weight:bold;font-family:courier;">grep "^[+-]{0.1}[0-9]{1,}$" numbers2.dat | more</span><br><br>'''NOTE:''' most likely, there were '''NO results'''. 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) 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.<br><br>
# Reissue the above pipeline command using '''egrep''' instead of ''grep'':<br><span style="color:blue;font-weight:bold;font-family:courier;">egrep "^[+-]{0,1}[0-9]{1,}$" numbers2.dat | tee better-number1.txt | more</span><br><br>You should have noticed that the command worked correctly this time because you used the '''egrep''' command.<br><br>'''NOTE:''' In extended regular expressions, the '''?''' symbol can be used to represent the '''{0,1}''' repetition symbols<br>and the '''+''' symbol can be used to represent the '''{1,}''' repetition symbols<br><br>
# Reissue the above pipeline command using '''egrep''' instead of ''grep'':<br><span style="color:blue;font-weight:bold;font-family:courier;">egrep "^[+-]?[0-9]+$" numbers2.dat | tee better-number2.txt | more</span><br><br>You should have seen the same results, but the extended regular expression required less typing.<br><br>