Created Regular Expressions.md

tutorials/learnshell.org/en/Regular Expressions.md

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+Regular Expressions
+--------
+Regular expression can be used as a powerful tool for pattern matching and text manipulation.
+Let's start with the basics and then move to more advanced concepts:
+
+Basic Characters:
+
+- Any single character is matched literally.
+- `.` (dot) matches any single character except newline.
+
+```bash
+echo "cat dog" | grep "c.t"  # Matches "cat"
+```
+
+Character Classes:
+
+- `[...]` matches any single character within the brackets.
+- `[^...]` matches any single character not within the brackets.
+
+```bash
+echo "cat dog" | grep "[cd]at"  # Matches "cat"
+echo "cat dog" | grep "[^c]at"  # Matches "dat" in "dog"
+```
+
+Predefined Character Classes:
+
+- `\d` matches any digit (equivalent to `[0-9]`)
+- `\w` matches any word character (equivalent to `[a-zA-Z0-9_]`)
+- `\s` matches any whitespace character
+
+```bash
+echo "abc123" | grep -E "\d+"  # Matches "123"
+```
+
+Anchors:
+
+- `^` matches the start of a line
+- `$` matches the end of a line
+
+```bash
+echo -e "start\nend" | grep "^end"  # Matches "end" at the start of a line
+```
+
+Quantifiers:
+
+- `*` matches 0 or more occurrences
+- `+` matches 1 or more occurrences
+- `?` matches 0 or 1 occurrence
+- `{n}` matches exactly n occurrences
+- `{n,}` matches n or more occurrences
+- `{n,m}` matches between n and m occurrences
+
+```bash
+echo "a aa aaa" | grep -E "a{2,}"  # Matches "aa" and "aaa"
+```
+Alternation:
+
+- `|` acts like an OR operator
+
+```bash
+echo "cat dog" | grep -E "cat|dog"  # Matches both "cat" and "dog"
+```
+
+Grouping and Capturing:
+
+- `(...)` groups expressions and creates a capturing group
+
+```bash
+echo "catdog" | grep -E "(cat|dog)+"  # Matches "catdog"
+```
+
+Backreferences:
+
+- `\1`, `\2`, etc., refer to captured groups
+
+```bash
+echo "catcat" | sed -E 's/(cat)\1/\1/'  # Replaces "catcat" with "cat"
+```
+
+Exercise
+--------
+In this exercise your task will be to write a regex that will match any line that starts and
+ends with the same word
+
+Tutorial Code
+-------------
+```
+#!/bin/bash
+
+# Chnage the REGEX variable to match the pattern
+REGEX="YOUR_REGEX_HERE"
+
+
+# Don't change the below code
+TEST=("CAT DOG" "TEA TEA" "RED BLUE" "DOG DOG" "SHELL SHELL" "BEEF BEEF" "TEA COFFEE" "SHELL BASH" "RED GREEN" "CAT FISH")
+
+for i in "${TEST[@]}" ; do
+    if [[ ! $i =~ $REGEX ]]; then
+        echo "Validation Failed for $i"
+    else
+        echo "Validation Passed for $i"
+    fi
+done
+```
+Expected Output
+---------------
+Validation Failed for CAT DOG
+Validation Passed for TEA TEA
+Validation Failed for RED BLUE
+Validation Passed for DOG DOG
+Validation Passed for SHELL SHELL
+Validation Passed for BEEF BEEF
+Validation Failed for TEA COFFEE
+Validation Failed for SHELL BASH
+Validation Failed for RED GREEN
+Validation Failed for CAT FISH
+
+Solution
+--------
+```
+#!/bin/bash
+
+# Chnage the REGEX variable to match the pattern
+REGEX="^(\w+) \1$"
+
+
+# Don't change the below code
+TEST=("CAT DOG" "TEA TEA" "RED BLUE" "DOG DOG" "SHELL SHELL" "BEEF BEEF" "TEA COFFEE" "SHELL BASH" "RED GREEN" "CAT FISH")
+
+for i in "${TEST[@]}" ; do
+    if [[ ! $i =~ $REGEX ]]; then
+        echo "Validation Failed for $i"
+    else
+        echo "Validation Passed for $i"
+    fi
+done
+```