Regular Expressions

1
Regular Expressions

• ashishFA_at_cse

2
Where can I use

• Powerful in
• - Search , search and replace, text processing
• Text Editors vi, editplus
• Programming languages - perl, Java
• Grep,awk

3
Before RegEx

• Wildcard
• .txt
• My_report.doc
• Here the indicates any number of any characters.

4
!
Regular expressions (RegEx) tend to be easier to
write than they are to read
5
What is RegEx

• a regular expression-- a pattern that describes
  or matches a set of strings
• Matched text chunk of text which matches the
  regular expression.
• catrn
• Matches car, can, cat
• Editplus is used throughout this presentation
  as tool to demonstrate the regular expressions

6
Structure of RegEx

• Made up of normal characters and metacharacters.
• Metacharacters special function
• . \ \( \) ?
• means end of line
• start of line

7
Literal match

• RegEx cat will match the word cat
• It will also match words like concatenation ,
  delicate, located, modification
• It is not desired sometimes ?
• solution

8
Matching

• Match the space before and after cat
• cat
• ? Still problem

9
Character class

• Want to search in or on ..
• So searching RegEx ion will match in and on
  both
• used to specify a set of character to
  select from.
• a-h indicates set of all characters from a to
  h
• 4-9A-T

10
Character class

• It can also contain individual characters as
  acV5y0
• 0-9 ?
• 0-90-9 ?
• 180-90-9?

11
Example

• set of vowels
• aeiou
• set of consonents
• bcdfghjklmnpqrstvwxyz
• Consider matching words which start with 2 vowels
  and end with consonant
• aeiouaeioubcdfghjklmnpqrstvwxyz ?
• aeiouaeioubcdfghjklmnpqrstvwxyz

12
Negation

• The absence of any character or set of character
  can be shown using symbol
• ab8 means not a , but b , but not 8
• c-p means any character other than c..p
• tion select all words ending with ion but
  with not before it

13
Start/End of line

• indicates start of line
• indicates end of line
• Example
• search lines starting with I
• Use RegEx I
• search lines ending ending with is
• Use RegEx is

14
match

• . Any character match
• e.e match all strings where first letter is e
  and last is e.
• Try e.e
• If you want only words to be searched then change
  the query to
• ea-ze

15
Repeated match

• match the previous character or
  character-class zero or more times
• be will match sequence of zero or more e
  preceded by b
• similar to
• Only difference is that it matches sequence of
  one or more.

16
Selecting a number

• Single digit 0-9
• When single digit is repeated zero or more times
  it is a number.
• (digit)repeat
• 0-9
• 0-9 ?
• \0-9

17
Selecting a word

• Word is composed of alphabets
• A word is a-z
• A word in all capital letters ??
• A word starting with capital letter

18
Alternate match

• symbol is used to specify alternate match
• Search (above)(below)

19
Search

• Day Words
• a-zday
• a-zday
• - A-Za-zday

20
Escaping Special meaning

• How to match (, ) or
• To match the characters which are used as
  Metacharacter, \ is added before them as an
  escape character.
• i.e. to match ( write \( and to match period .
  write \.

21
Search patterns

• has, have, had
• not, nt
• ((have)(had)(has))
• (( )(n't)( not ))
• ((have)(had)(has))(( )(n't)( not ))

22
(No Transcript)
23
References

• Editplus help pages
• http//gnosis.cx/publish/programming/regular_expre
  ssions.html
• OReilly - Mastering Regular Expressions
• Google regular expression tutorial

24
Thank you !