Basic Regular expressions Interval Regular expressions Extended regular expressions Summary
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Basic Regular expressions
Some of the commonly used commands with Regular expressions are tr, sed, vi and grep. Listed below are some of the basic Regex. Let’s see an example. Execute cat sample to see contents of an existing file
Search for content containing letter ‘a’.
‘^‘ matches the start of a string. Let’s search for content that STARTS with a
Only lines that start with character are filtered. Lines which do not contain the character ‘a’ at the start are ignored. Let’s look into another example –
Select only those lines that end with t using $
Interval Regular expressions
These expressions tell us about the number of occurrences of a character in a string. They are Example: Filter out all lines that contain character ‘p’
We want to check that the character ‘p’ appears exactly 2 times in a string one after the other. For this the syntax would be:
Note: You need to add -E with these regular expressions.
Extended regular expressions
These regular expressions contain combinations of more than one expression. Some of them are: Example: Searching for all characters ‘t’
Suppose we want to filter out lines where character ‘a’ precedes character ‘t’ We can use command like
Brace expansion
The syntax for brace expansion is either a sequence or a comma separated list of items inside curly braces “{}”. The starting and ending items in a sequence are separated by two periods “..”. Some examples:
In the above examples, the echo command creates strings using the brace expansion.
Summary:
Regular expressions are a set of characters used to check patterns in strings
They are also called ‘regexp’ and ‘regex’
It is important to learn regular expressions for writing scripts
Some basic regular expressions are:
Some extended regular expressions are:
Some interval regular expressions are:
The brace expansion is used to generate strings. It helps in creating multiple strings out of one.
