Regular Expressions

Regular expressions let you describe patterns in text. They are used everywhere in the shell, especially with grep, sed, and awk.

Matching with grep

grep searches lines that match a pattern:

echo "apple banana cherry" | grep apple

Prints the whole line because it contains apple:

apple banana cherry

Anchors

^ anchors a match to the start of a line, $ to the end:

echo "apple" | grep "^a"   # lines starting with 'a'
echo "apple" | grep "e$"   # lines ending with 'e'

Character classes

Square brackets match any one character in the set:

echo "apple" | grep "a[p]le"   # the 'p' is literal here

Ranges are written with a dash, e.g. [0-9] for any digit and [a-z] for any lowercase letter.

Quantifiers

* means "zero or more", + means "one or more", and ? means "zero or one":

echo "color" | grep -E "colou?r"   # matches color or colour
echo "colour" | grep -E "colou?r"

Exercise

Complete the grep below so that it matches only the line that starts with the letter a.