As I understood, RegExs are very useful for general work. Most of the following regular expressions (RegEx)s can be run on the macOS terminal, where you can get the great value of command line tools that have no value without RegExs (grep, sed and so on). In addition, I’ve used some popular tools to explain complex operations later in the document, which have been referenced under the footnotes.



macOS grep or ggrep

For the macOS, the default grep (FreeBSD version) is minimal. Using the macOS standard grep command, you cannot run all the bash commands expressed in this blog post. Therefore install the grep from the homebrew:

brew install grep

and use the command ggrep instead of grep. For the help

ggrep --help

NOTE: The improved grep is ggrep.

For testing purposes, download the texts of Shakespeare1 as a zip file after you extract the zip file.

Find a text in a file

To find the word in the TXT file:

egrep --color the hamlet_TXT_FolgerShakespeare.txt

This will show you something similar to the following output.

image-20220304192546992

You can see the the is highlighted.

Find in the folder

Locate the file in the file system where every file shows the lines that contain text henry.

find . -exec egrep -H 'Henry IV' {} \; 2>/dev/null

List the file which has the seach

For example, case-sensitive match:

egrep --color 'the|is' hamlet_TXT_FolgerShakespeare.txt

multiple words in the search

For case insensitive matches, use the -i option.

egrep --color -i 'england' hamlet_TXT_FolgerShakespeare.txt

case-intensive-search

To implement the Enhanced Regular Expressions (ERE) dialect, use grep -E(grep -P say PCRE, for MacOS use the perl in the terminal), and egrep is the shortened form of that.

If you want a collection of text to match, you can use either alternation | or an external text file with the words.

echo 'england' >>  search_keywords.txt
echo 'ambassadors' >>  search_keywords.txt
echo 'gives' >> search_keywords.txt

Search command is

egrep --color -f search_keywords.txt hamlet_TXT_FolgerShakespeare.txt

To show the lines do not match (negative of the above), use v option:

egrep --color -v -i -f search_keywords.txt hamlet_TXT_FolgerShakespeare.txt

Character Class

You can create complex search key expressions using a class of characters. To create a character class, use the [] in the search. For example

egrep --color -i '[abc]' hamlet_TXT_FolgerShakespeare.txt

Character class example

The complement of the above is [^abc].

complement of the character class

For example:

Generic Character classes

Generic Class
\d [0-9]
\D [^0-9]
\w [a-zA-Z0-9]
\W [^a-zA-Z0-9]
\s [\t\n\r\f]
\S [^\t\n\r\f]

For example

echo 'Hello Ojitha 1234' | grep --color -i '\D'

Generic character class example

Only the words are selected.

Select only the part of the searching word

Posix Character Classes

This has been created to simplify the character classes. The syntax is [[:CLASS:]]. Use the ^ as the complement [[:^CLASS:]].

Only Perl regex supports the following classes.

Use Perl in the macOS terminal

Posix Characte Class Description
alnum [a-zA-Z0-9] letters and digits
word [a-zA-Z0-9] word characters
alpha [a-zA-Z] Letters
digit [0-9] Digits
lower [a-z] lower case letters
upper [A-Z] upper case letters
space [\t\n\f\r] White space
grep --color -E  '[[:upper:]]' hamlet_TXT_FolgerShakespeare.txt

Or

perl  -ne 'print if /[[:^word:]]/' hamlet_TXT_FolgerShakespeare.txt

Or

ggrep --color -P  '[[:upper:]]' hamlet_TXT_FolgerShakespeare.txt

another example to search 12 and non other digits:

not numbers except 1 and 2

Quantifiers

In the regex expression, we want to quantify how many characters we want to match, for example.

Quantifier Description
? 0 or 1
* 0 or more
+ 1 or more
{n} n
{n,} match n or more
{n,m} match n thorugh m

For example

echo 'my 20 birtday party @ bay' |egrep --color '\d{2}\s\w{3}'

can be visualized2 as:

quantifier example

The output of the above regex is

Use_of_quantifier

In the Perl regex

echo 'my 20 Birtday party @ Bay' |grep --color -E '[[:upper:]]{1}'

output is

Use Quntifiers with Posix classes

See the following example:

Quantifiers are greedy

Quantifiers are greedy, the consume as much as they can.

Inline Modifiers

Modifier Description
(?x) Embed whitespace
(?i) Case insensitive match
(?s) Single line mode
(?m) Multi line mode, here \A start if the string and \Z end of the string.

For example, although I specify the Posix class lower for lower letters, when you specify modifier (?xi), it shows all the words ignoring the Upper case letters.

echo 'my 20 Birtday party @ BAY' |ggrep --color -P '(?xi) [[:lower:]]'

Use of modifiers

Example use of multi-lines, starting (^) with Hello:

echo 'Hello
Ojitha
How are you' | ggrep --color -Pz '(?xm) ^Hello .* '

image-20220305154939892

In the above bash command, -z option allows the input data to be a sequence of lines.

See the modification above for single line modifier:

echo 'Hello
Ojitha
How are you' | ggrep --color -Pz '(?xs) ^Hello .* '

image-20220305162321037

All the lines are selected because \n new line has been tread as another character in the single line(s) mode.

example of embedding white space

Embedding whitespace

In the above regex the whitespace to search is given as \x20.

Multi-line as single line

In the above regex, the new line is treated as \n because s treated the two lines as an one line.

if you use m instad of s:

Search in the multi-lines

Bounding

Bonding regex does not match characters, but they specify where in the string the regex is to be matched.

Bounding Meaning
^ Beginning
$ End of the string or before \n
\A begining of string
\Z end of string
\b begining or end of a word
\B complement of begining or end of a word
echo "the pen is my brother's" | ggrep -P --color '\bthe'

use of bounding \a

As shown above, brother’s is not selected because the word the is not the beginning of that word according to the bounding \b.

But, if you use bounding \B:

echo "the pen is my brother's" | ggrep -P --color '\Bthe'

image-20220305170512740

alternation

Either match this or that: This has already been introduced in the beginning.

echo 'one and two are numbers' > test.txt
perl -pe 's/one|two/digit/' test.txt
digit and two are numbers

You can use alternation with bounds as follows:

echo 'I have borther
but I have no sisters
and any other ...' | ggrep -P --color '^I|the'

bounds with alternation

If you use in the group

echo 'I have borther
but I have no sisters
and any other ...' | ggrep -P --color '^(I|and)'

alternation with group

Lazy Quantifiers

Not going to consume greedy but minimally.

Quntifier Description
*? zero or more minimal
+? one or more minimal
?? zero or one minimal
{n}? n times minimal (n is numeric)
{n,}? n times or more minimal (n is numeric)
{n,m}? n through m minimal (n is numeric)

As shown in the following screenshot, the first regex greedly consume and match the first letter a to last letter t. In the second regex, it matches the first and up to the closest letter t.

Lazy select closest

In the first grep command there is only one mactch. In the second, there are two maches: ‘azy is import’ and ‘ant’.

For example see the difference in the outputs when you apply lazy quantifier:

Lazy Quantifier example -1

another example:

Lazy Quantifier example -2

Possessive Quantifier

Possessive quantifiers are like normally greedy quantifiers, the important difference is that the possessive quantifiers do not backtrack (go back) unlike greedy quantifiers.

Quantifier Description
*+ zero or more possessive
++ one or more possessive
?+ zero or one possessive

Capture

You can create capture groups using () as follows:

(.+)\.(png|gif)

The graphical representation3 is

Capture Group Diagram

The target string match as follows:

Capture group output

For non-capture groups, use the ?: as follows:

(.+)\.(?:png|gif)

The above regex matched the string but hasn’t captured the second group

non capture

non capture group

you can reference the captured group later in the expression by the positional value

reference capture by its position

echo 'Hello
ball
narrow
mirrrror' | ggrep -P --color '(.)\1'

Captureing

and visualisation4back reference

echo 'Hello
memo
mississippi
rivier
lala
mama mia' | ggrep -P --color '(.)(.)\1\2'

image-20220305225344362

This is same as above

echo 'Hello
memo
mississippi
rivier
lala
mama mia' | ggrep -P --color '(..)\1'

notice (..)\1.

Word followed the same word:

echo ' the song
I like ba baba black ship
wonder why' | ggrep -P --color '\b(\w+)\s\1'

repeat the same word twice

Good practical example is HTML tag matching such as <(\w+)>.*?<\/\1> as shown in the visualisation:

image-20221221155344369

html tag validation

You can name the capture group as follows. In the editor5, you can substitute the HTML em tag in markdown bold text. More similar to boundary tokens.

image-20221221115615187

Using Sed

sed -E 's/([a-z]*) (\d*)/text: \1, digits: \2/'
text: hello, digits: 123

For example, non capture using perl:

first group is non capture

Lookarounds

Lookarounds are regex expression conditions that are not captured as a part of the match.

  Postive Negative
Lookahead T (?=c): Capture T which statisfy the condition c after it. Negation of T (?!c)
Lookbehind (?<=c) T: Capture T which satisfy the condtion c before it. Negation of(?<!c) T

Examples:

     
Postive Lookahead image-20221221163509915 image-20221221163752540
Negative Lookahead image-20221221164838168 image-20221221164510708
Positive lookbehind image-20221221191545911  
Negative Lookbehind image-20221221191659565  

NOTE: Python support only fixed-width lookarounds. Indermine quantifiers such as *,?,+ are not allowed.

Example of Postive Lookahead where I want to remote date_parse and date_format function from the following line:

GROUP BY "date_trunc"('day', "date_parse"("date_format"(convert_timezone('Australia/Sydney', table-a.field-x), '%d/%m/%Y %H:%i:%s'), '%d/%m/%Y %H:%i:%s'))

The regular expression to find is "date_parse"\(.*(convert_timezone.*?\)).*(?=\)) and the replacement in VSCode, is $1. The Postive Lookahead has been used to avoid the selection of last ).

VSCode

You can use VSCode to capture the CSV column and modify it. For example,

Before After
before run replace after run replace

Look

VSCode support the lookarounds as well.

VSCode lookarounds

Stream editor

By default, sed uses the BRE dialect. The -E option uses ERE. The sed command can modify a file inline using the -i option.

touch test.txt
echo 'Hi ojitha' > test.txt
sed -i '.bak' 's/ojitha/OJ/' test.txt

image-20220304201448235

Text mactches using &:

echo 'Hello ojitha' | sed 's/oj.*/(&)/g'
Hello (ojitha)

Merge multiple lines to group of lines

For example you want to couple two lines to one line int he following text:

11111,
22222,
33333,
44444,
55555,
66666,
77777,
88888,
99999,

You have to follow 4 steps in the visual studio code

  1. search regex: (^(\d+,\n){2}) and replace regex: --$1-- and the result is
    --11111,
    22222,
    ----33333,
    44444,
    ----55555,
    66666,
    ----77777,
    88888,
    --99999,
    

    In the above code, 2 lines are selected to compose as an one line.

  2. search regx: \n and replace regex: empty and the result is
    --11111,22222,----33333,44444,----55555,66666,----77777,88888,--99999,
    
  3. search regex: ---- and replace regex: \n and the retults is
    --11111,22222,
    33333,44444,
    55555,66666,
    77777,88888,--99999,
    
  4. search : -- and replace: empty and the result is
    11111,22222,
    33333,44444,
    55555,66666,
    77777,88888,99999,