Week 4 - Regular Expressions

Why regular expressions?

Regex
Allows searching using a range of characters rather than just the keyword
More Powerful than keyword search

Regex:

A Pattern of special characters used to match strings in a search
Typically made up from special characters called meta-characters
Used throughout the web, software and Linux
- grep, egrep, sed, and awk
- Python

Meta-characters:

. - Will match any character except a new line character
[-] - Will match one character from a range of characters on its left and right
* - Will match zero or more of the preceding character
? - Will match zero or one of the preceding characters
+ - Will match one or more of the preceding characters
^ - Beginning of the line
$ - End of the line
\char - Escape the meaning of the char following it
[^] - One of the character not in the set
\b - Beginning/End of the word anchor
| - OR grouping
x{m} - Repetition of the character x, m times
x{m,} - Repetition of the character x, at least m times
x{m,n} -Repetition of the character x between m and n times e.g.

[A-H] -> [ABCDEFGH]
[A-Z] -> Any Uppercase alphabetic
[0-9] -> Any digit
[[a] -> [ or a
[0-9\-] -> Any Digit or hyphen
[^AB] -> Any character except A or B
[A-Za-z] -> Any alphabetic
[^0-9] -> Any character except a digit
[]a] -> ] or a
[^/^] -> Anything except ^

Anchors:

Anchors tell where the next character in the pattern must be located in the text data.

^ - Beginning of line
$ - End of line
\b - Beginning of word
\b - End of word
\A - Beginning of string
\z - Very end of string
\Z - End of string (or before optional line break)
\G - Beginning of string or end of previous match

e.g.

[a-zA-Z0-9_] - are word characters \b4\b - will match 4 but not 44 \bis\b - will match is in the string “This island is beautiful.” but nothing else

Back References:

We can use back references in regex to re-use previously matched patterns in the regex

The pattern is defined by brackets (regex), and then the matched text can be recalled from a buffer using: \<buffer-number>
buffer-number increments with each back reference

([a-c])x\1x\1 - Matches axaxa , bxbxb and cxcxcx where the reference is in bold

Alternation / OR operator: |

The operator | is used to match one or more alternatives.

UNIX|unix → matches “UNIX” or “unix”
Ms|Miss|Mrs → matches “Ms” or “Miss” or “Mrs”
cat|dog|mouse|fish → matches those words

Repetition Operator: {…}

The repetition operator specifies that the expression immediately before the repetition may be repeated.

A{3,5} → matches “AAA”, “AAAA”, or “AAAAA”
BA{3,5} → matches “BAAA”, “BAAAA”, or “BAAAAA”

Basic repetition forms:

{m} - matches previous atom exactly m times
{m,} - matches previous atom m or more times
{,n} - matches previous atom n times or less

e.g.

CA{5} -> CAAAAA
CA{3,} -> CAAA, CAAAA ,CAAAAA , ...
CA{,2} -> C, CA, CAA

Group Operator:

In the group operator, when a group of characters is enclosed in parentheses the next operator applies to the whole group, not only the previous characters.

e.g.

A(BC){3} -> ABCBCBC
(F(BC){2}G){2} -> FBCBCGFBCBCG