Regular expression

One can use the regular expressions to match in vi, grep, sed and awk.

[a-d] – Match one character with in a-d i.e. a, b, c, d
[^a-d] – Match one character not in the range a-d
\<test\> – Match whole word test
test\> – Match words that ends with test
\<test\> \1ing – Match following text “test testing”, \1 maps to first tag i.e \(\)
x\{5,\} – Match at least 5 occurrences of x
x\{5,9\} – Match between 5 to 9 times occurrences of x
^test – Looks for test at the beginning of a line
test$ – Looks for test at the end of the line
^test$ – Looks for test on a line by itself
th.t – “.” matches one character i.e. 4 letters has th + any character and ends with t. Example: this, that are valid matches
\. – Look for period, using “\” one can escape metacharacters

Search and replace:
:s/\(square\) and \(fair\)/\2 and \1/ – searches for “square and fair” and replaces it with fair and square

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