Regex - details

 

1. What is a Regular Expression?

A regular expression is a pattern used to match text.

Example text:

file_2024-01-10.log
file_2024-02-11.log
notes.txt

Regex can extract only the date.

Example:

[0-9]{4}-[0-9]{2}-[0-9]{2}

Matches:

2024-01-10
2024-02-11

---

2. Where Regex is Used in Bash

Common Bash tools using regex:

Command	Purpose
grep	search text
sed	modify text
awk	pattern scanning
[[ =~ ]]	bash regex matching

Example:

grep '[0-9]' file.txt

Find lines containing numbers.

---

3. Basic Regex Characters

Literal Match

Regex normally matches exact characters.

Example:

cat

Matches:

cat

But not:

cats
scatter

---

4. The Dot (.)

.

Means:

any single character

Example:

c.t

Matches:

cat
cut
cot
c9t

But NOT:

ct
cart

---

5. Character Classes

[abc]

Match any one character inside brackets

Example:

gr[ae]y

Matches:

gray
grey

---

[a-z]

Range match.

Example:

[a-z]

Matches any lowercase letter

Examples:

a
m
z

---

[A-Z]

Uppercase letters.

---

[0-9]

Numbers.

Example:

[0-9][0-9]

Matches:

12
45
99

---

6. Negated Character Class

[^abc]

Means:

anything except a,b,c

Example:

[^0-9]

Matches:

a
B
_

But not:

5
7

---

7. Anchors

Anchors match position, not characters.

---

Beginning of line ^

^hello

Matches:

hello world

But not:

say hello

Example in bash:

grep '^ERROR' logfile

---

End of line $

world$

Matches:

hello world

But not:

world hello

Example:

grep '\.log$'

Matches files ending with .log.

---

8. Repetition Operators

These define how many times something repeats.

---

* (zero or more)

ab*

Matches:

a
ab
abb
abbb

---

+ (one or more)

ab+

Matches:

ab
abb
abbbb

Not:

a

Note: + works in extended regex.

---

? (zero or one)

colou?r

Matches:

color
colour

---

9. Exact Repetition {}

Specify exact counts.

---

Exactly N

[0-9]{4}

Matches:

2024
1999

---

Range

[0-9]{2,4}

Matches:

12
123
1234

---

Minimum

[0-9]{2,}

Matches:

12
123
12345

---

10. Groups ()

Parentheses create groups.

Example:

(abc)+

Matches:

abc
abcabc
abcabcabc

---

11. Alternation |

Acts like OR.

Example:

cat|dog

Matches:

cat
dog

---

12. Escaping Special Characters

Some characters are special:

. * + ? ^ $ ( ) [ ] { } |

To match them literally use \.

Example:

Match .log

\.log

---

13. Bash Built-in Regex Matching

Bash has regex using [[ ]].

Example:

file="log_2024-10-01.txt"

if [[ $file =~ [0-9]{4}-[0-9]{2}-[0-9]{2} ]]
then
    echo "date found"
fi

---

14. Capturing Groups in Bash

Example:

file="backup_2024-01-10.tar"

if [[ $file =~ ([0-9]{4})-([0-9]{2})-([0-9]{2}) ]]
then
    echo "Year: ${BASH_REMATCH[1]}"
    echo "Month: ${BASH_REMATCH[2]}"
    echo "Day: ${BASH_REMATCH[3]}"
fi

BASH_REMATCH array contains matched groups.

---

15. Real Shell Example

Extract log date.

Example file names:

app-2024-10-01.log
app-2024-10-02.log
error.log

Script:

for f in *.log
do
    if [[ $f =~ ([0-9]{4}-[0-9]{2}-[0-9]{2}) ]]
    then
        echo "Date: ${BASH_REMATCH[1]}"
    fi
done

Output:

Date: 2024-10-01
Date: 2024-10-02