Zintis Regular Expression Cheat

Two sites exist that make testing your regular expressions easy:

1. https://regexr.com and
2. https://regex101.com

Match a [ by escaping it with a \ so matching [5] woule be \[5\]

ipv4 addresses:

([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})
but you do not need the round brackets around [0-9]{1,3}
[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}
this can be rewritten as:

([0-9]{1,3}\.){3}[0-9]{1,3}


if there there are spaces before and after the address:
\b(?:[0-9]{1,3}\.){3}[0-9]{1,3}\b

But that would match address lid 787.888.910.666 so we can be more accurate:
\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]?)\b

alias addr="ip addr | egrep '([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})'"

They will gobble up the biggest block that matches the expression.

Regexp are by default greedy so a regexp of ^.*MOS by default will match everything from the beginning of the line to the last MOS found. i.e.

Nowadays a chicken leg is a rare dish MOS1 : The depth of a well is hard to fathom MOS2

then ^.*MOS will match:

Nowadays a chicken leg is a rare dish MOS1 : The depth of a well is hard to
fathom MOS

But if your regexp is ^.*?MOS it is not greedy and you would match the first MOS it found. eg:

Nowadays a chicken leg is a rare dish MOS

So sed 's/^.*MOS//' would get you 2

and sed 's/^.*?MOS//' would get you 1 : The depth of a well is hard to fathom MOS

Regular expressions module, re, needs to be imported before use. It is part of the standard python library, so all pythons should have re.

Use is re.search() to see if a string matches a regexp, similar to find() method for strings

use re.findall() to extract portiosn of a string that match the regexp, which is similar to running find() followed by slicing var[5:10]

In the search part of a regex, expressions wrapped in brackets () are stored in variables that can be recalled using $1 for the first set of brackets, $2 for the 2nd, etc. The regex inside the brackets are called a "capturing group" They can be used in the replacement pattern of the grep or other utilities.

• $n includes the last substring matched by the nth capturing group
• ${ name } includes the last substring matched by the named group designated by {?<name> ) in the replacement string
• $$ includes a single literal "$" in the replacement string.
• $& includes a copy fo the entire match in the replacement string
• $` includes all the text of the input string before the match in the replacement string
• $' includes all the text of the input string after the match in the replacement string
• $+ includes the last group captured in the replacement string
• $_ includes the entire input string in the replacement string

Example:

• String searched: "@gmail.com addr–"
• Regexp: (\w+)\W+(\w+)

$` –> "@" $& –> "gmail.com addr" $& –> $& –> $& –> $& –> $& –>

egrep "^([^f]|f([^o]|o([^r]|$)|$)|$).*$" cars.txt will match all llines that do NOT contain for

So file cars.txt

plym    fury    77      73      2500
chevy   nova    79      60      3000
ford    mustang 65      45      17000
volvo   gl      78      102     9850
ford    ltd     83      15      10500
Chevy   nova    80      50      3500
fiat    600     65      115     450
honda   accord  81      30      6000
ford    thundbd 84      10      17000
toyota  tercel  82      180     750
chevy   impala  65      85      1550
ford    bronco  83      25      9525

then egrep "^([^f]|f([^o]|o([^r]|$)|$)|$).*$" cars.txt will result in the output:

plym    fury    77      73      2500
chevy   nova    79      60      3000
volvo   gl      78      102     9850
Chevy   nova    80      50      3500
fiat    600     65      115     450
honda   accord  81      30      6000
toyota  tercel  82      180     750
chevy   impala  65      85      1550