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

Date Search

^[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)
*/

Number >= 42 Search

^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

Dollar $ Search

^\$?\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 x modifier.

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