Regular expressions (regex) are patterns used to match character combinations in strings. They allow searching and replacing text. Common uses include validation, parsing, extraction and finding/replacing patterns. While powerful, regexes have limitations and should not try to do everything in one expression. Additional resources provide guidance on proper regex usage.

2. And are they contagious?

3. There is no official standard for
regular expressions, so no real
definition.
Simply put, you can call it a
text pattern to search and/or
Easy peasy!
replace text.

4. Perl programming language
Perl-compatible
.NET
Java
JavaScript
…
What, no cherry flavour?

5. Back to grammar school!

6. a matches any occurrence of that character
Jack is a boy.
cat matches
About cats and dogs.

7. square bracket [
backslash
caret ^
dollar sign $
period or dot .
vertical bar or pipe symbol |
question mark ?
asterisk or star *
plus sign +
opening round bracket (
closing round bracket )
opening curley bracket {

8. Special characters are reserved for special use.
They need to be preceded by a backslash if you want to
match them as literal characters.
This is called escaping.
If you want to match 1+1=2 the correct regex is 1+1=2

9. tab t
carriage return r
line feed n
beginning of line ^
end of line $
word boundary b

10. If regular expressions are Unicode enabled you can
search any character using the Unicode value.
Depending on syntax: u0000 or x{0000}
Hard space u00A0 or x{00A0}
® sign u00AE or x{00AE}
...

11. Quantifiers allow you to specify the number of
occurrences to match against
X? X, once or not at all
X* X, zero or more times
X+ X, one or more times
X{n} X, exactly n times
X{n,} X, at least n times
X{n,m} X, at least n but not more than m times

12. The regex colou?r matches both colour and color.
You can also group items together by using brackets:
Nov(ember)? will match Nov and November
The regex a+ is the same as a{1,} and matches a or aaaaa
The regex w{3} matches www.qa-distiller.com

13. Simply place the characters you want to match between
square brackets.
If you want to match an a or an e, use [ae]. You could
use this in gr[ae]y to match either gray or grey.
A character class matches only a single character, the
order is not important
You can also use ranges. [0-9] matches a single digit
between 0 and 9

14. Typing a caret after the opening square bracket will negate
the character class.
q[^u] means: "a q followed by a character that is not a u".
It will match the q and the space after the q in
Iraq is a political quagmire.
but not the q of quagmire because it is followed by the
letter u

15. d digit [0-9]
w word character [A-Za-z0-9_ ]
s whitespace [ trn]
Negated versions
D not a digit [^d]
W not a word character [^w]
S not a whitespace [^s]

16. The dot matches a single character, without caring what
that character is.
The regex e. matches
Houston, we have a problem

17. If you want to search for cat or dog, separate both options
with a vertical bar or pipe symbol:
cat|dog matches
Are you sure you want a cat?
You can add more options like this:
green|black|yellow|white

18. Which of the following completely matches regex a(ab)*a
1) abababa
2) aaba
3) aabbaa
4) aba
5) aabababa

19. Which of the following completely matches regex ab+c?
1) abc
2) ac
3) abbb
4) bbc
5) abbcc

20. Which of the following completely matches regex a.[bc]+
1) abc
2) abbbbbbbb
3) azc
4) abcbcbcbc
5) ac
6) asccbbbbcbcccc

21. Which of the following completely matches regex
(very )+(fat )?(tall|ugly) man
1) very fat man
2) fat tall man
3) very very fat ugly man
4) very very very tall man

22. Still awake?

23. Positive lookahead: X(?=X)
Match something that is followed by something
Yamagata(?= Europe) matches
Yamagata Europe, Yamagata Intech Solutions
Negative lookahead: X(?!X)
Match something that is not followed by something
Yamagata(?! Europe) matches
Yamagata Europe, Yamagata Intech Solutions

24. Positive lookbehind: (?<=X)X
Match something following something
(?<=a)b matches
thingamabob
Negative lookbehind: (?<!X)X
Match something not following something
(?<!a)b matches
thingamabob

25. Round brackets create a backreference.
You can use the backreference with a backslash + the number of the
backreference.
The regex Java(script) is a 1ing language matches
Javascript is a scripting language
The regex (Java)(script) is a 2ing language that is not the same as 1
matches
Javascript is a scripting language that is not the same as Java

26. Use the regex b(w+) 1b to find doubled words.
Ze streelde haar haar in in de auto.
With exceptions:
b(?!haarb)(w+) 1b
Ze streelde haar haar in in de auto.

27. You want to add brackets around step numbers:
This is step 5 from chapter 1. Continue with step 45 from page 15.
Use the regex ([sS]tep) (d+) to find all instances.
Replace it by 1 (2)
Or alternatively (?<=[sS]tep )d+ by (0)

28. Powerful, for individual text-based files
More powerful, batch operations, command line
No back references
RegEx Text File Filter
RegEx search
Very limited
Powerful, called GREP

29. Some people, when confronted with a problem, think
"I know, I'll use regular expressions.“
Now they have two problems.
-> Do not try to do everything in one uber-regex
-> Regular expressions are not parsers

30. http://www.regular-expressions.info