My notes on Regular Expression

Non Technical:

Regular : Happening Frequently

Expression: Word or group of words

Each character in regular expression may be one of two types:

Normal Characters

Special Characters

Normal Characters:

Normal Characters are Any character except [\^$.|?*+{}

The regular expression “cat” contains three normal characters, it will match the following patterns: “concatenate”, “the cat ran away”

By default , regular expressions are case sensitive. (treated differently depending on whether it is in capitals or lowercase text)

Special Characters:

Certain Characters reserved for special use

[,\,^,$.,|,?,*,+,(,)

Also called as metacharacters

To use as Normal, escape them with a backslash

Character Classes or Character Set

[ ] groups characters into a character set

Will match any single character from the set

E.g. <<gr[ae]y>> matches “gray” or “grey”

Order of characters inside character class do not matter

Use hyphen(‘-’) to specify a range of characters

E.g.: <<[0-9]>> matches a single digit between 0 and 9

E.g:: <<[0-9a-fA-F]>> matches single hexadecimal digit

Negated Character Classes        

‘^’ after opening bracket will negate the character class

Matches any character not in the character class.

Shorthand character classes

\d -> [0-9]

\w -> [A-Za-z0-9_]

\s -> [\s\t]

\D -> [^\d]

\W -> [^\w]

\S -> [^\s]

Dot(.) Character

Most commonly used metacharacter

Matches a single character without caring what that character is

Exception is ‘\n’

Dot in short is for <<[^\n]>>

Anchors

“^” matches the beginning of the line

“$” matches the end

Alternation

Match a single regular expression out of possible regular expressions

E.g.: <<cat|dog>>

Repetition

* -> Match preceding character zero or more times

+ -> Match preceding character one or more times.

{m} -> m repetitions

{m,n} -> m to n repetitions

Question Mark(?) makes the preceding token optional

E.g.: <<colou?r>> matches both “colour” and “color”