Regular Expressions

Regular Expression: Set of characters that specify a pattern.

• Used when you want to search for lines containing a particular pattern.
  • We search for lines because most UNIX utilities operate on ASCII files line-by-line.
• Can only search for patterns on a single line.
  • They can’t search for patterns that stat on one line and end on another.
• Powerful and flexible.
  • e.g., searching for words of a certain size, searching for words with n or more variables that ends with “s”, etc.
• Can match a string in more than one place (not just once per line).

Tangent: “Regular” comes from term used to describe grammar and formal languages (“regular languages”).

Remember: Regular expressions are just patterns, what they do is determined by how a utility uses them.

Two Types of Regular Expressions:

1. “Basic”
   • Used by most programs.
2. “Extended”
   • Used by a few utilities.

Regular Expression Metacharacters

Note: This is just a summary, examples and detailed descriptions are further down.

Regular Expressions v.s. Shell Metacharacters

Regular expressions and shell metacharacters are often confused with each other, but are very different.

Shell Metacharacters:

• Square brackets are similar.
• Asterisk acts similar to regular expressions (but not identical).
• Expanded before shell passes the argument to the program.

Remember: Remember to wrap regular expressions in quotes to prevent them from being erroneously expanded by the shell!

Structure of a Regular Expression

Three Important Parts of a Regular Expression:

1. Anchors: Specify position of pattern in relation to a line of text.
2. Character Sets: Match one or more characters in a single position.
3. Modifiers: Specify how many times the previous character set is repeated.

Examples: Regular expressions (matches are underlined)

cks

UNIX rocks
UNIX sucks
UNIX is okay

• Regex that matches for the string “cks” (no anchor or modifier).

apple

Scrapple from the apple

• Regex that matches for “apple”.
  • Note how there are multiple matches.

^#*

UNIX rocks
UNIX sucks
UNIX is okay

• Useless regex with an anchor (^), character set (#), and modifier (*).
  • ^: Indicates beginning of the line.
  • #: Matches the pound symbol (#).
  • *: Specified that the previous character set can appear any number of times, including zero.

b[eor]at

I tried to eat bratwurst on the boat, but was beaten to the punch by a seagull

• Regex that matches for “b[eor]at”
  • Note: Regex character classes ([]) behave similarly to, but aren’t the same as the [] file substitution wildcard metacharacter.

b[^eo]at

I tried to eat bratwurst on the boat, but was beaten to the punch by a seagull

• Regex that matches for “b[^eo]at”.
  • Note: We can negate character classes with [^] syntax.

Utilities using Regular Expressions

Character Classes

Named Character Classes

Commonly-used character classes can be referred to by name.

• e.g., alpha, lower, upper, alnum, digit, punct, cntrl
• Important for portability across languages.

Format: [:name:]

More on Character Classes

Examples: Some more examples

• [aeiou] will match any of the characters a, e, i, o, or u
• [kK]orn will match korn or Korn

1. Ranges can also be specified in character classes

Examples: Using ranges

• [1-9] is the same as [123456789]
• [abcde] is equivalent to [a-e]

2. You can also combine multiple ranges

• Note that the - character has a special meaning in a character class, but only if it is used within a range
  • For example, [-123] would match the characters -, 1, 2, or 3

3. You can mix explicit characters and character ranges.

Note; The characters ] and - do not have a special meaning if they directly follow a [

• e.g., []0-9] will match for any digit or the symbol ]
• Tip: You can use the backslash (\) to escape ] and - like you do for shell metacharacters.

Exercises: Reading Regular Expressions

Instructions: Read the following regular expressions and determine what they do/match.

Questions:

1. ^T[a-z][aeiou]
2. [-0-9]
3. [0-9\-b\]]
   • Hint: The \’s are escaping some special characters.
4. U[nN][iI]X

Answers:

1. Matches for 3-character strings that begin with T, are followed by any lowercase letter, and end with a vowel.
2. Matches for any character that is a digit or the symbol -
3. Matches for any character that is a digit, the symbol -, the letter b, or the symbol ].
4. Matches for the strings UniX, UnIX, UNiX, or UNIX.

More on Anchors

Anchors: Used to match at the beginning (^) or end ($) of a line (or both).

• Regular expressions examine the text between the separators.
  • The end of line character isn’t included in the block of text that’s searched because it is a separator.
• When using ^, place the ^ at the beginning of the expression for it to be interpreted as an anchor.
• When using $, place the $ at the end of the expression for it to be interpreted as an anchor.

Examples: Using anchors

^bingus

bingus bingus bingus

• Regex that matches for the string “bingus” at the start of a line

bingus$

bingus bingus bingus

• Regex that matches for the string “bingus” at the end of a line

More on Modifiers

Matching a Single Character (.)

. is a special character that, by itself, matches any character (except the end of line character).

Repetition in Regular Expressions (*)

* is a special character is used to define zero or more occurrences of the single regular expression preceding it.

• e.g., 0* matches zero or more 0s, [0-9]* matches zero or more of any digit. -Only acts as a modifier if it follows a character set.

Examples: Using *

ya*y

I got mail, yaaaaaay!

oa*o

For me to poop on.

a.*e

Scrapple from the apple.

• A match will be the longest string that satisfies the regular expression.
  • Notice how apple and apple from the also matched the regex, but were shorter.

Repetition Ranges (\{ and \})

To specify a minimum and maximum number of repeats, put the minimum and maximum between \{ and \}.

• e.g., \{1,3\} -Only acts as a modifier if it follows a character set.

Note: Unlike shell metacharacters, backslash turns on the special meaning for { and }.

Notation (\{ \}): Ways to specify a range of repetitions for the preceding regular expression.

• \{n\}: Specify exactly n occurrences
• \{n, \}: Specify at least n occurrences
• \{n,m\}: Specify at least n occurrences but no more than m occurrences

Examples: Using \{ \}

Regular Expression	Matches
.\{0,\}	Exactly the same as .*
a\{2,\}	Exactly the same as aaa*
*	Any line with an asterisk
\*	Any line with an asterisk
\\	Any line with a backslash
^*	Any line starting with an asterisk
^A*	Any line
^A\*	Any line starting with an “A*”
^AA*	Any line if it starts with one “A”
^AA*B	Any line with one or more “A”’s followed by a “B”
^A\{4,8\}B	Any line starting with 4, 5, 6, 7 or 8 “A”’s followed by a “B”
^A\{4,\}B	Any line starting with 4 or more “A”’s followed by a “B”
^A\{4\}B	Any line starting with “AAAAB”
\{4,8\}	Any line with “{4,8}”
A{4,8}	Any line with “A{4,8}”

Repetitions of Subexpressions (\( and \))

\( \) groups parts of an expression into subexpressions.

• Subexpressions are treated like a single character.
• Lets you apply * and \{\} to more than just the previous character.

Note: Unlike shell metacharacters, backslash turns on the special meaning for ( and ).

Examples: Grouping expressions with \(\)

abc*

• Matches ab, abc, abcc, abccc, …

\(abc\)*

• Matches abc, abcabc, abcabcabc, …

\(abc\)\{2,3\}

• Matches abcabc or abcabcabc

Backreferences (\(,\) and \1)

Backreferences let you remember what you found and see if the same pattern occurred again:

1. Mark part of a pattern with \( and \).
   • Each occurrence of \( starts a new pattern.
2. Recall the remembered pattern with \ followed by a single digit.
   • You can have 9 different remembered patterns.

Examples: Using backreferences

\([a-z]\)\1

• Match for two consecutive identical letters.

\([a-z]\)\1\1

• Match for three consecutive identical letters.

\([a-z]\)\([a-z]\)[a-z]\2\1

• Match for a 5-letter palindrome (e.g., radar).

Extended Regular Expression

egrep and awk use extended regular expressions.

Differences from Basic Regular Expressions:

• Special characters preceded by a backslash no longer have their special meaning.
  • “\{”, “\}”, “\<”, “\>”, “\(”, “\)”, “\”digit
• The character ? matches 0 or 1 instances of the character set before.
  • Basic regular expression counterpart: \{0,1\}
• The character + matches 1 or more copies of the character set.
  • Basic regular expression counterpart: \{1,\}
• New metacharacters: (, | and ) to make a choice of patterns.
  • e.g., egrep '^(From|Subject): "/usr/spool/mail${USER}" prints all From: and Subject: lines from incoming mail.

Exercises: Reading Regular Expressions

Instructions: Read the following regular expressions and determine what they do/match.

Questions:

1. [a-zA-Z_][a-zA-Z_0-9]*
2. \$[0-9]+(\.[0-9][0-9])?
3. (1[012]|[1-9]):[0-5][0-9] (am|pm)
4. <[hH][1-4]>

Answers:

1. Matches for variable names in C
2. Matches for dollar amount with optional cents
3. Matches for time of day
4. Matches for HTML headers (<h1>, <H1>, <h2>, …)