Advanced Regular Expressions in Grep Command with 10 Examples – Part II

In our previous regular expression part 1 article, we reviewed basic reg-ex with practical examples.

But we can do much more with the regular expressions. You can often accomplish complex tasks with a single regular expression instead of writing several lines of codes.



When applying a regex to a string, the regex engine will start at the first character of the string. It will try all possible permutations of the regular expression at the first character. Only if all possibilities have been tried and found to fail, will the regex engine continue with the second character in the text.

The regex will try all possible permutations of the regex, in exactly the same order. The result is that the regex-directed engine will return the leftmost match.

In this article, let us review some advanced regular expression with examples.

Example 1. OR Operation (|)

Pipe character (|) in grep is used to specify that either of two whole subexpressions occur in a position. “subexpression1|subexpression2” matches either subexpression1 or subexpression2.

The following example will remove three various kind of comment lines in a file using OR in a grep command.

First, create a sample file called “comments”.

$ cat comments This file shows the comment character in various programming/scripting languages ### Perl / shell scripting If the Line starts with single hash symbol, then its a comment in Perl and shell scripting. ' VB Scripting comment The line should start with a single quote to comment in VB scripting. // C programming single line comment. Double slashes in the beginning of the line for single line comment in C.

The file called “comments” has perl,VB script and C programming comment lines. Now the following grep command searches for the line which does not start with # or single quote (‘) or double front slashes (//).

$ grep -v "^#\|^'\|^\/\/" comments This file shows the comment character in various programming/scripting languages If the Line starts with single hash symbol, then its a comment in Perl and shell scripting. The line should start with a single quote to comment in VB scripting. Double slashes in the beginning of the line for single line comment in C.

Example 2. Character class expression

As we have seen in our previous regex article example 9, list of characters can be mentioned with in the square brackets to match only one out of several characters. Grep command supports some special character classes that denote certain common ranges. Few of them are listed here. Refer man page of grep to know various character class expressions.

[:digit:] Only the digits 0 to 9 [:alnum:] Any alphanumeric character 0 to 9 OR A to Z or a to z. [:alpha:] Any alpha character A to Z or a to z. [:blank:] Space and TAB characters only.

These are always used inside square brackets in the form [[:digit:]]. Now let us grep all the process Ids of ntpd daemon process using appropriate character class expression.

$ grep -e "ntpd\[[[:digit:]]\+\]" /var/log/messages.4 Oct 28 11:42:20 gstuff1 ntpd[2241]: synchronized to LOCAL(0), stratum 10 Oct 28 11:42:20 gstuff1 ntpd[2241]: synchronized to 15.11.13.123, stratum 3 Oct 28 12:33:31 gstuff1 ntpd[2241]: synchronized to LOCAL(0), stratum 10 Oct 28 12:50:46 gstuff1 ntpd[2241]: synchronized to 15.11.13.123, stratum 3 Oct 29 07:55:29 gstuff1 ntpd[2241]: time reset -0.180737 s

Example 3. M to N occurences ({m,n})

A regular expression followed by {m,n} indicates that the preceding item is matched at least m times, but not more than n times. The values of m and n must be non-negative and smaller than 255.

The following example prints the line if its in the range of 0 to 99999.

$ cat number 12 12345 123456 19816282 $ grep "^[0-9]\{1,5\}$" number 12 12345

The file called “number” has the list of numbers, the above grep command matches only the number which 1 (minimum is 0) to 5 digits (maximum 99999).

Note: For basic grep command examples, read 15 Practical Grep Command Examples.

Example 4. Exact M occurence ({m})

A Regular expression followed by {m} matches exactly m occurences of the preceding expression. The following grep command will display only the number which has 5 digits.

$ grep "^[0-9]\{5\}$" number 12345

Example 5. M or more occurences ({m,})

A Regular expression followed by {m,} matches m or more occurences of the preceding expression. The following grep command will display the number which has 5 or more digits.

$ grep "[0-9]\{5,\}" number 12345 123456 19816282

Note: Did you know that you can use bzgrep command to search for a string or a pattern (regular expression) on bzip2 compressed files.

Example 6. Word boundary (\b)

\b is to match for a word boundary. \b matches any character(s) at the beginning (\bxx) and/or end (xx\b) of a word, thus \bthe\b will find the but not thet, but \bthe will find they.

# grep -i "\bthe\b" comments This file shows the comment character in various programming/scripting languages If the Line starts with single hash symbol, The line should start with a single quote to comment in VB scripting. Double slashes in the beginning of the line for single line comment in C.

Example 7. Back references (

)

Grouping the expressions for further use is available in grep through back-references. For ex, \([0-9]\)\1 matches two digit number in which both the digits are same number like 11,22,33 etc.,

# grep -e '^\(abc\)\1$' abc abcabc abcabc

In the above grep command, it accepts the input the STDIN. when it reads the input “abc” it didnt match, The line “abcabc” matches with the given expression so it prints. If you want to use Extended regular expression its always preferred to use egrep command. grep with -e option also works like egrep, but you have to escape the special characters like paranthesis.

Note: You can also use zgrep command to to search inside a compressed gz file.

Example 8. Match the pattern “Object Oriented”

So far we have seen different tips in grep command, Now using those tips, let us match “object oriented” in various formats.

$ grep "OO\|\([oO]bject\( \|\-\)[oO]riented\)"

The above grep command matches the “OO”, “object oriented”, “Object-oriented” and etc.,

Example 9. Print the line “vowel singlecharacter samevowel”

The following grep command print all lines containing a vowel (a, e, i, o, or u) followed by a single character followed by the same vowel again. Thus, it will find eve or adam but not vera.

$ cat input evening adam vera $ grep "\([aeiou]\).\1" input evening adam

Example 10. Valid IP address

The following grep command matches only valid IP address.

$ cat input 15.12.141.121 255.255.255 255.255.255.255 256.125.124.124 $ egrep '\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)' input 15.12.141.121 255.255.255.255

In the regular expression given above, there are different conditions. These conditioned matches should occur three times and one more class is mentioned separately.

If it starts with 25, next number should be 0 to 5 (250 to 255) If it starts with 2, next number could be 0-4 followed by 0-9 (200 to 249) zero occurence of 0 or 1, 0-9, then zero occurence of any number between 0-9 (0 to 199) Then dot character

For the 1st part of this article, read Regular Expressions in Grep Command with 10 Examples – Part I

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