�ݺ�ߣ

Chapter 3
Introduction to Regular Expressions
1
Dr. Hadeel Alazzam
Scripting Programming
1

2
? Regular Expression
? Commands in use
? grep and egrep
? Regular Expression Metacharacters
? Grouping
? Brackets and Character Classes
? Back References
? Quantifiers
? Anchors and Word Boundaries
? Practical Examples
Outline

3
RegularExpression
? Regular expressions (regex) are a powerful method for
describing a text pattern to be matched by various tools.
? There is only one place in bash where regular
expressions are valid, using the =~ comparison in the [[
compound command, as in an if statement.
? Regular expressions are a crucial part of the larger toolkit
for commands like grep, awk, and sed in particular.

4
RegularExpressionvs. PatternMatching
? Pattern matching is used by the shell commands such
as the ls command.
? Regular expressions are used to search for strings of
text in a file by using commands, such as the grep
command.
? The use of regular expressions is generally associated
with text processing.

5
CommandsinUse
? grep: The grep command searches the content of the files for a given
pattern and prints any line where the pattern is matched.
? To use grep, you need to provide it with a pattern and one or more
filenames (or piped data).
? Common command options:
? -c: Count the number of lines that match the pattern.
? -E: Enable extended regular expressions.
? -f: Read the search pattern from a provided file. A file can contain
more than one pattern, with each line containing a single pattern.
? -i: Ignore character case.
? -l: Print only the filename and path where the pattern was found.
? -n: Print the line number of the file where the pattern was found.
? -p: Enable the Perl regular expression engine.
? -R, -r: Recursively search subdirectories.

6
CommandsinUse
? In general, grep is used like this:
? grep options pattern filenames
? To search the /home directory and all subdirectories for files containing
the word password, regardless of uppercase/lowercase distinctions:

7
grepandegrep
? The grep command supports some variations, notably extended syntax
for the regex patterns
? There are three ways to tell grep that you want special meaning on
certain characters:
1. by preceding those characters with a backslash.
2. by telling grep that you want the special syntax (without the need for
a backslash) by using the -E option when you invoke grep.
3. by using the command named egrep, which is a script that simply
invokes grep as grep �CE so you don��t have to.
? The only characters that are affected by the extended syntax are? + { | (
and ).

Regular Expression Metacharacters
? Regular expressions are patterns that are created
using a series of characters and metacharacters.
? Metacharacters such as the questions mark (?) and
asterisk (*) have special meaning beyond their literal
meanings in regex.
? The 7 lines of frost.txt file will be used in the next
slides examples.

? 1 Two roads diverged in a yellow wood,
? 2 And sorry I could not travel both
? 3 And be one traveler, long I stood
? 4 And looked down one as far as I could
? 5 To where it bent in the undergrowth;
? 6
? 7 Excerpt from The Road Not Taken by
Robert Frost

10
? The ��.�� Metacharacter:
? The period (.) represents a single wildcard character.
? It will match on any single character except for a newline.
? If you want to treat this metacharacter as a period character rather
than a wildcard, precede it with a backslash (.) to escape its special
meaning.
? If we try to match on the
pattern T.o, the first line of
the frost.txt file is returned
because it contains the
word Two
? Regex patterns are also case
sensitive, which is why line
3 of the file is not returned
even though it contains the
string too

11
? The ��?�� Metacharacter:
? The question mark (?) character makes any item that precedes it
optional.
? It matches it zero or one time.
? This pattern will match on any three-character sequence that begins with T and ends
with o as well as the two-character sequence To.
? Note that we are using egrep here.
? We could have used grep �CE,
? or we could have used ��plain�� grep with a slightly different pattern: T.?o, putting
the backslash on the question mark to give it the extended meaning.

12
? The ��*�� Metacharacter:
? The asterisk (*) is a special character that matches the preceding item
zero or more times.
? It is similar to ?, the main difference being that the previous item may
appear more than once.
? The .* in the preceding pattern allows any number of any character to
appear between the T and o.
? Thus, the last line also matches because it contains the pattern The
Ro.

13
? The ��+�� Metacharacter:
? The plus sign (+) metacharacter is the same as the * except it requires
the preceding item to appear at least once.
? The preceding pattern specifies one or more of any character to
appear in between the T and o.
? The first line of text matches because of Two �� the w is one character
between the T and the o.
? The second line doesn��t match the To, as in the previous example;
rather, the pattern matches a much larger string �� all the way to the o
in undergrowth.
? The last line also matches because it contains the pattern The Ro.

14
Grouping
? We can use parentheses to group characters.
? Among other things, this allows us to treat the characters appearing inside
the parentheses as a single item that we can later reference.
? Here, we use parentheses and the Boolean OR operator (|) to create a
pattern that will match on line 3.
? Line 3 as written has the word traveler in it, but this pattern would match
even if traveler was replaced by the word stranger.

15
Brackets and Character Classes
? The square brackets, [ ] , are used to define character classes and lists of
acceptable characters.
? Using this construct, you can list exactly which characters are matched at
this position in the pattern.
? This is particularly useful when trying to perform user-input validation.
? As shorthand, you can specify ranges with a dash, such as [a-j].
? These ranges are in your locale��s collating sequence and alphabet.
? The pattern [a-j] will match one of the letters a through j.

16
? Table 3-1 provides a list of common examples when using character
classes and ranges.
? Be careful when defining a range for digits; the range can at most go from 0 to 9.
For example, the pattern [1-475] does not match on numbers between 1 and 475;
it matches on any one of the digits (characters) in the range 1�C4 or the character 7
or the character 5.

17
? There are also predefined character classes known as shortcuts.
? These can be used to indicate common character classes such as
numbers or letters.

18
? Note that the shortcuts are not supported by egrep.
? In order to use them, you must use grep with the -p option.
? That option enables the Perl regular expression engine to support the
shortcuts.
? Note: -p (small letter)

19
? Other character classes (are valid only within
the bracket syntax, as shown in Table 3-3.
? They match a single character, so if you need
to match many in a row, use the * or + to get
the repetition you need.
? To use one of these classes, it has to be inside
the brackets, so you end up with two sets of
brackets.
? This will match any line with an X followed by
any uppercase letter or digit. It would match
these lines:

20

21
Back References
? Regex back references are one of the most powerful and often confusing
regex operations.
? Consider the following file, tags.txt:
? Suppose you want to write a regular expression that will extract any line
that contains a matching pair of complete HTML tags.
? The start tag has an HTML tag name; the ending tag has the same tag
name but with a leading slash. <div> and </div> are a matching pair.
You can search for these by writing a lengthy regex that contains all
possible HTML tag values, or you can focus on the format of an
? HTML tag and use a regex back reference, as follows:

22
Back References
? In this example, the back reference is the 1 appearing in the latter part of
the regular expression.
? It is referring back to the expression enclosed in the first set of
parentheses, [A-Za-z]*, which has two parts.
? The letter range in brackets denotes a choice of any letter, uppercase or
lowercase.
? The * that follows it means to repeat that zero or more times.
? Therefore, the 1 refers to whatever was matched by that pattern in
parentheses.
? If [A-Za-z]* matches div, then the 1 also refers to the pattern div.

23
Back References
? You can have more than one back reference in an expression and refer
to each with a 1 or 2 or 3 depending on its order in the regular
expression
? A 1 refers to the first set of parentheses, 2 to the second, and so on
? Note that the parentheses are metacharacters; they have a special
meaning.
? If you just want to match a literal parenthesis, you need to escape its
special meaning by preceding it with a backslash, as in sin([0-9.]*) to
match expressions like sin(6.2) or sin(3.14159).

24
Quantifiers
? Quantifiers specify the number of times an item must appear in a string.
? Quantifiers are defined by curly braces { }.
? For example, the pattern T{5} means that the letter T must appear
consecutively exactly five times.
? The pattern T{3,6} means that the letter T must appear consecutively
three to six times.
? The pattern T{5,} means that the letter T must appear five or more times.

25
Anchors and Word Boundaries
? You can use anchors to specify that a pattern must exist at the beginning
or the end of a string.
? The caret (^) character is used to anchor a pattern to the beginning of a
string.
? For example, ^[1-5] means that a matching string must start with one of
the digits 1 through 5, as the first character on the line.
? The $ character is used to anchor a pattern to the end of a string or line.
? For example, [1-5]$ means that a string must end with one of the digits 1
through 5.
? In addition, you can use b to identify a word boundary (i.e., a space).
? The pattern b[1-5]b will match on any of the digits 1 through 5, where
the digit appears as its own word.

End
27
Dr. Aryaf Al-adwan, Autonomous Systems Dept 27

�ݺ�ߣ

Chapter 3: Introduction to Regular Expression

Recommended

More Related Content

Similar to Chapter 3: Introduction to Regular Expression (20)

Recently uploaded (20)

Chapter 3: Introduction to Regular Expression

Editor's Notes