Regular Expression Dr. Tran, Van Hoai Faculty of Computer Science and Engineering HCMC Uni. of...

Regular Expression

Dr. Tran, Van Hoai

Faculty of Computer Science and Engineering HCMC Uni. of Technology

[email protected]

Dr. Tran, Van Hoai2007

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Text pattern

Used for text-processing utilities Text-pattern = normal characters +

metacharacters = regular expression Metacharaters in regular expressions are

different from those in file name expansion


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Example (1)

grep [A-Z]* script*.shmeansgrep a.txt abc script1.sh script2.sh

grep "[a-z]*" script*.shmeans to find the pattern "[a-z]*" in

"script*.sh" Good and safe solutions are "" and ''


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Metacharacter sets Depends on usage context

searchingreplacing

Also depends on programs Different engines

PerlPHP .NET regular expression libraryJava JDK


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Searching patterns (1)

Character Pattern

. single character, except newline

* any number of characters immediately preceding it

^ the following regex at the beginning

[ ] any one of the enclosed characters, which can be given in range (-). "^" right after "[" means not

{n,m} a range of occurences of regex preceding it. {n} matches exactly n occurrences. {n,} matches at least n occurrences. {n,m} matches occurrences between n and m

\ turn off special meaning

\b word boundary


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Searching patterns (2)

Character Pattern

\{n,m\} a range of occurences of regex preceding it. {n} matches exactly n occurrences. {n,} matches at least n occurrences. {n,m} matches occurrences between n and m

+ one or more instances of preceding regex

? zero or one instances of preceding regex

| alternation

( ) apply match to an enclosed group of regex


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Example (2)

Pattern What does it match?

bag bag

^bag bag at the beginning of line

bag$ bag at the end of line

^bag$ only bag on the line

[Bb]ag Bag or bag


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Example (3)


b[aeiou]g second letter is vowel

b[^aeiou]g second letter is consonant (or uppercase, symbol)

b.g second letter is any

^\. any line begins with a dot

^\.[a-z][a-z] ?

^[^\.] ?

bugs* bug, bugs, bugss,etc.


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Example (4)


"word" ?

"*word"* ?

[A-Z][A-Z]* one or more uppercase letters

^\. any line begins with a dot

^\.[a-z][a-z] ?

^[^\.] ?

bugs* bug, bugs, bugss,etc.


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Example (5)


floating point number

java identifier

java simple arithmetic expression


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Replacing patterns (1)

Character Pattern

\ turn off special meaning

\n reuse the text matched by the nth subpattern previously saved by $ and $. Numbered from 1 to 9

& text match search pattern

~, % reuse previous replacement pattern

\u convert first character of replacement pattern to uppercase

\U convert entire replacement pattern to uppercase

\l, \L same; to lowercase

\e turn off previous \u or \l

\E turn off previous \U or \L


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Example (6)

Command Result

s/.*/( & )/ add space and parentheses

s/.*/mv & &.old/ ?

/^$/d delete blank lines (vi, g/^$/d for all lines)

%s/ */ /g turn one or more spaces into one space

%s/.*/\L&/ lowercase entire file

%s/yes/No/g replace yes to No

%s/Yes/~/g replace yes to No (previous replacement)

s/$F$$ORTRAN$/\1\L\2/g

FORTRAN to Fortran


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Applications

Pattern Matched text

grab a specific HTML tag

[0-9]\{1,3\}\. ???? IP address

Email address

Valid dates (day-month-year)

WeWe, does not match Wee

<TAG\b[^>]*>$.*?$</TAG>

[A-Z0-9._%-]+@[A-Z0-9.-]+\.[A-Z]{2,4}


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