Regular Expressions, Searching, Replacing, Building
What is a Regular Expression
A regular expression (
regex) is a pattern that matches a set of strings, consisting of operators, constructs literal characters and meta-characters.
📌 grep command supports tree regex syntaxes. Check some basics usage here.
Regular expressions are powerful, used in areas like search engines, programming languages and text processing applications.
Examples
Check how to find or validate an Email Address - RFC 5322 Official Standard
More examples here emailregex.com
# Find all email addresses in a file using grep
grep -E -o "\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,6}\b" filename.txt
# Python
r"(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)"
# Javascript
/^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/
#MySQL
SELECT * FROM `users` WHERE `email` NOT REGEXP '^[A-Z0-9._%-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$';Searching with RegEx
There a 4 primary components into a regular expression:
character classes
quantifiers and alternation
anchors
roots and anchors
^[A-Z][a-z]{2,}\s+[0-3]?[1-9],\s+[12]?[0-9]{0,3}$
/*
^ = ANCHOR - match the beginning of the string or line (with the multiline flag m)
[A-Z][a-z] = CHARACTER CLASSES - range matching upper/lower case letters
{2,} = QUANTIFIER - match 2 or more of the preceding token
\s = whitespace
+ = QUANTIFIER - match 1 or more of the preceding whitespace
* = QUANTIFIER - match 0 or more of the preceding token
? = QUANTIFIER - optional, match 0 or 1 of the preceding token
| = QUANTIFIER - alternation, match 1 token or another
$ = ANCHOR - match the end of the string or line (with the multiline flag m)
*/
^4[2-9]|[5-9]\d|[1-9]\d{2,}$
/*
\d = match any digit character (0-9) = [0-9]
*/
grep -E "^4[2-9]|[5-9]\d|[1-9]\d{2,}$" numbers.txt
42
# grep (posix) doesn't have the \d shortcut for digits
grep -E "^4[2-9]|[5-9][1-9]|[1-9][1-9]{3,}$" numbers.txt
42
55
95
^\$?\s*[1-9][0-9]{0,2}(,?[0-9]{3})*(\.[0-9]{2})?$
/*
\$ = ESCAPED CHAR - match a "$" character
*/
Replacing with RegEx
Replacing text with regular expressions varies between implementations.
Examples with the regexr site tool:
Tips on Building RegEx
Regular expressions are very powerful but are not appropriate for every problem.
Regular expressions are greedy (they'll match as much as they can).
Add and
?after*or+too make the match lazy (match the minimum possible)
Don't write an entire regex all at once.
Build a piece - test it - repeat
Use multiple, simpler, smaller expressions
Test with valid and invalid data - ensure the regex matches only what you want it to match.
Add comments using
xmodifier.
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