Fish Shell:
Using regular expressions

How to:

While Fish Shell itself does not have a built-in command for regex, it effectively uses external commands like grep, sed, and awk that support regex, allowing you to incorporate regex operations in your scripts.

Basic Pattern Matching with grep

Search for lines in a file that match a pattern:

grep '^[0-9]+' myfile.txt

This command finds lines starting with one or more digits in myfile.txt.

Extracting & Replacing with sed

Extract phone numbers from a file:

sed -n '/\([0-9]\{3\}\)-\([0-9]\{3\}\)-\([0-9]\{4\}\)/p' contacts.txt

Replace all occurrences of “foo” with “bar” in data.txt:

sed 's/foo/bar/g' data.txt

Using string for Basic Regex

Fish Shell’s string command supports simple regex operations like match and replace:

Match a pattern in a string:

echo "fish 3.1.2" | string match -r '3\.[0-9]+\.[0-9]+'

Output:

3.1.2

Replace digits following ‘fish’ with ‘X.X.X’:

echo "Welcome to fish 3.1.2" | string replace -ra '([fish]+\s)[0-9\.]+' '$1X.X.X'

Output:

Welcome to fish X.X.X

Advanced Matching with awk

Print the second column of data where the first column matches a specific pattern:

awk '$1 ~ /^a[0-9]+$/ {print $2}' datafile

This command looks for lines in datafile where the first column starts with an “a” followed by one or more digits and prints the second column.

By integrating these external commands, Fish Shell programmers can harness the full power of regular expressions for complex text manipulation tasks, enhancing the shell’s native capabilities.