10. Regular Expression (Part 1)

Topics:

Regular expression
- Text Strings
Metacharacters
- Metacharacters that match a single character
- Metacharacters that are anchors
- Metacharacters used for repetition

Regular Expression

A regular expression (abbreviated regex) is
- a series of characters
- that you give to certain filters or utilities
- to describe what you want to find in a line of input
In a regular expression
- most characters have their literal meaning (‘a’ means you’re looking for the letter ‘a’, space means you’re looking for a space character)
- some characters have special meaning (similar to the shell metacharacters). These are also called metacharacters
How the regular expression search engine works
- walk the line of input, starting from the beginning of the line
- look for a match of the series of characters in the regular expression
- If there is no match by the end of the line, the search engine concludes that the line is not a match
- As soon as there is a match somewhere in the line, the search engine stops and concludes that the line is a match

Text Strings

A text string with letters, numbers, punctuations and spaces is the simplest regular expression. Each character in the string takes on its literal meaning
When you use a simple text string as a regular expression, you look for an exact match of the text string, anywhere in the line
Examples:
- egrep ‘cis18a’ input_file
  - Lines that have cis18a or mycis18a or cis18abc will be sent out to screen.
  - Lines with: Cis18a or CIS18A or cis 18a or cis18 will not match
- egrep ‘cis 18a ’ input_file
  - All lines that have cis, followed by a space, followed by 18a, followed by a space anywhere in the line will be sent out to screen.
  - All other lines will not be sent out to screen

Metacharacters

In addition to simple text strings, regular expressions usually have metacharacters
These metacharacters make the regular expression more flexible (and thus more useful than text strings) because they allow for a range of possibilities in the search pattern.
- You can specify a match of ‘linux’ or ‘Linux’, for example, in the following topics are the metacharacters in the extended set of regular expression. The extended set contains more metacharacters in it than the original set of regular expression.
Any character that is not a metacharacter is literal character
For this notes we use the utility egrep to work with the extended set of regular expression

Metacharacters that match a single character

[characters] any one character within the [ ] is a match
[^characters] any one character not within the [ ] is a match
. any one character that is not the end-of-line character is a match (end-of-line character is the character created by hitting the enter key)

Examples

egrep ‘[Ll]inux’ inputFile
- Any line with Linux or linux will match
egrep ‘file[^123]’ inputFile
- Any line with file followed by a 1 or 2 or 3 will not match.
- The following will not match: file1 or file2.3 or file380
egrep ‘lab[12][345]’ inputFile
- Any line with lab13 or lab14 or lab15 or lab23 or lab24 or lab25 will match
egrep ‘CIS..A’ inputFile
- Any line with CIS, followed by any 2 characters, followed by A will match.
- This can be CIS18A or CIS33A or CISbcA or even CIS A (2 spaces between S and A), but not CISA or CISbcdA

Metacharacters that are anchors

^ marks the beginning of the line
- This character dictates the position of the next character in the regex.
- If used, ^ has to be the first character in the regex.
$ marks the end of the line
- This character matches the position of the previous character in the regex.
- If used, $ has to be the last character in the regex.

Examples

egrep ‘a’ inputFile
- Any line with a anywhere in the line will match
egrep ‘^a’ inputFile
- Any line with a at the beginning of the line will match
egrep ‘a$’ inputFile
- Any line with a at the end of the line will match

Metacharacters used for repetition

When a character in a regex needs to appear multiple times in a row in the match, it is easier to use the repeat metacharacters, rather than typing in a character multiple times
- {n} n is a number, the previous character must appear n times
- {n,m} n and m are numbers, the previous character must appear a minimum of n times and a maximum of m times
- ? The previous character can appear 0 or 1 time
- + the previous character must appear at least 1 time
- * the previous character can appear 0 or more time
The metacharacters {n,m}, + and * use greedy matching.
- This means the search engine will match as many characters in the input line as possible.
- For example, if the input line is: baaaaabc
  - the regex ‘a+’ will match all 5 a’s
  - the regex ‘a{2,6}’ will match all 5 a’s
The * metacharacter can be counter-intuitive with what it matches
- For example, in the previous example line: baaaaabc
- the regex ‘a*’ will match the letter b at the beginning of the line. This is because ‘a*’ means 0 a or as many a’s as possible, and the first b happens to match 0 a.
- the regex ‘ba*’ will match the first b and then all 5 a’s
More examples:
- egrep ‘ab{5,9}c’ inputFile
  - Any line with a, followed by 5 up to 9 b’s, followed by c will match
- egrep ‘^ *[0-9]’ inputFile
  - Any line that starts with or without spaces in front, followed by at least 1 digit will match
- egrep ‘^[0-9]+$’ inputFile
  - Any line that has only digits (and no other type of characters) will match
  - Notice that using the anchors ^ and $ means we’re describing the whole line of text, from beginning to end. And in this case, between the beginning and the end of the line, we only allow 1 or more digits

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