An Introduction to Regular expressions

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