Match patterns like a pro

Regex or regular expressions are a way to match patterns in text, it is kind of like a language. The reason it is an important part of linux is that many programs in linux use regular experessions and by learning them you can become an advanced user.

Metacharacters

• These are some special characters used in regular expressions to match complex patterns.
• Some of these metacharacters are: ^, $, ., [ ], { }, -, ?, +, *, ( ), |, \ etc.
• Moving on we'll learn about these metacharacters one by one.

The caret (^) sign

• It is used to match the start of a line.
• For example if we want to match the line that starts with "Because", we can do this:
  • ^Because

The dollar ($) sign

• It is used to match the end of the line.
• For example if we want to match the line that ends with "bye", we can do this:
  • bye$

Match anything using (.)

• So the . character in regular expression can match any other character.
• For example if you are matching 3 letter words starting with "ma", like "man", "map", "mam", "may", "mac" etc. We can do this:
  • ma.
• To be honest a better way to search for 3 letter words starting with "ma", would be this:
  • \bma.\b
• \b matches the beginning or end of a word.

Create character classes ([ ]) and specify ranges (-)

• Character classes are a way to match a character from the given list of characters.
• For example:
  • [bch]atman can match "batman", "catman" or "hatman"
• The above regular expression does not match "bcfatman" as beginners sometimes misunderstand.
• You can also give ranges for the character classes using hyphen.
• Pre-built character classes:
  • \d - Matches any digit [0-9].
  • \D - Matches everything except digit [^0-9].
  • \w - Matches any character, digit or underscore [a-zA-Z0-9_].
  • \W - Matches everything except character, digit or underscore [^a-zA-Z0-9_].
  • \s - Matches space, tab, carriage return, line feed or form feed. [ \t\r\n\f].
  • \S - Matches everything except space, tab, carriage return or form feed [^ \t\r\n\f].
• Examples:
  • [a-zA-Z0-9]
  • [a-z0-9]
  • [A-H]atman
  • [a-z\-] - Matches lowercase characters and hyphen (-)

Match a number of characters at a time ({ })

• You can match some pattern a minimum, maximum or fixed number of times using braces.
• Let's create a regex for an IP address.
• An IP address is of the format:
  • someNumberBetween0-255.someNumberBetween0-255.someNumberBetween0-255.someNumberBetween0-255
• So we can say in regex it is:
  • [0-9] (minimum 1 to maximum 3), yes it will include numbers out the range 0-255 but let's get to that later.
  • So we can write the above expression as \d{1,3}
  • After the number we have a full stop but we need to escape it since fullstop is a metacharacter in regex.
    • So it becomes \d{1,3}\.
  • Now we can to repeat this whole pattern 3 times, so it becomes:
    • (\d{1,3}\.){3} Here we grouped the whole previous pattern using (), we'll learn about it later.
  • Now at last their is one more number which can be minimum 1 digit to maximum 3 digits.
    • (\d{1,3}\.){3}\d{1,3}
• Now if we want to be strict with the 0-255 range we can do something like this:
  • ((1?\d{1,2}|2[0-5]{2})\.){3}(1?\d{1,2}|2[0-5]{2})

Match zero or one times (?)

• You can match a character zero or one time using ?, it is helpful when a character may or may not exist in string.
• For example if we want to match a phone number it may or may not have + symbol in the beginning:
  • \+?91-\d{10}

Match one or more times (+)

• You can match a character one or more times using +, it is helpful when you want the character to be matched atleast once.
• So if we want to match "Baby shark do do do do...", we can do this:
• Baby shark (do\s?)+

Match zero or more times (*)

• You can match a character zero or more times using *.
• Example if we want to match any tag in html, we can do this:
  • <.*>

Grouping (( ))

• We can group parts of our regex to specify the number of times they should be matched or to output the groups seperately (possible in many programming languages).
• Like in the IP address example we looked before we grouped parts of it to match them a specific number of times using braces { }
• Examples:
  • ((1?\d{1,2}|2[0-5]{2})\.){3}(1?\d{1,2}|2[0-5]{2})
  • Baby shark (do\s?)+

Or operator (|)

• This is used when one of the specified pattern/string may be present but we're not sure which one.
• Example:
  • bat(man|woman) - This will match "batman" or "batwoman"
• The above example can also be done like this:
  • bat(wo)?man

Escape character (\)

• Many times we want to use characters like +, *, ?, $ or ^ as normal characters in our regex.
• To do that we can use \ to escape those metacharacters.
• Example:
  • \+1 \(\d{3}\) \d{3}\-\d{4} - Matches US phone numbers

Negation (^)

• When the caret symbol is used with character classes, it negates that character class.
• Example:
  • [^A-Z] - Matches everything except uppercase alphabets.

grep - global regular expression print

• This is a program that can use regular expression to find patterns in files.
• Usage: grep [OPTIONS] <pattern> [FILE...]
• Use the -i or --ignore-case flag to do a case-insensitive match.
• Use the -v or --invert-match flag to print lines without the pattern.
• Use the -l flag to only output file name of match.
• Use the -c flag for count of matches.
• Use the -n flag for number of line match was found on.
• Use the -P flag for perl style regex use.
• Use the -r flag for recursive search inside a directory.
• Use the -o flag to output only what matched.
• Examples:
  • grep -Pn "Baby shark (do\s?)+" babyShark.lrc
  • grep -ron "John Doe" ./dir/
  • grep -Pic "Bat(wo)?man" batmanChronicles.script

find command

• You can use the -regex flag in find command to do pattern matching.
• Example:
  • find . -regex '.*[^-_./0-9a-zA-Z].*'

locate command

• You can use the --regex flag in locate command to do pattern matching.
• Example:
  • locate --regex 'bin/(bz|gz|zip)'