Regex and Functions

Regex

The main uses for Regular Expressions (REs) are text searches and string manipulation. An RE matches a single character or a set of characters -- a string or a part of a string.

Regex metacharacters

'.' (dot) - Matches any single character except the newline character (\n).
'*' (star) - Matches zero or more occurrence of the immediately preceding character.
'<' - Matches the beginning of a word
'>' - Matches the ending of a word
'^' - Matches the beginning of a line
'$' - Matches the end of a line
'{m}' - Matches the exactly regex ‘m’
'{m,}' - Matches the at least regex ‘m’
'{m,n}' - Matches the preceding regex ‘m’ to ‘n’ times

Regex Metaclasses:

'[:alnum:]' - Alphanumeric characters; same as [a-zA-Z0-9]
'[:digit:]' - Digits; same as [0-9]
'[:punct:]' - Punctuation characters

The following regex will match all words end with ing:

<.*ing\>

The following regex will match all words which contain quu or quuu and such expression:

qu{2,}

Input:

domineering
joke
unite
meeting
test
improve
provide
tease
duck
truthful
striped
glamorous

Match all words starting with non-vowel and end with a vowel (Usage with grep).

cat lite.txt | grep "^[^aeiou].*[aeiou]$"

Output:

joke
provide
tease

Sed/Awk

Sed

Sed is a powerful text stream editor which can do insertion, deletion, search and replace.

Input:

cat
caat
caaat
caaaat
caaaaat

Replace consecutive multiple a with single a

sed 's/a\{2,\}/a/g' cats.txt

Output:

cat
cat
cat
cat
cat

Awk

Awk searches files for text containing a pattern and performs a specific action on that line/text.

Input:

A,20,30
B,10,5
C,25,40
D,40,10

Calculate total of the students

awk -F, 'BEGIN {printf "name \t sub1 \t sub2 \t total\n"} {printf "%s \t %s \t %s \t %s\n", $1, $2, $3,$2+$3}' marks.txt

Output

name 	 sub1 	 sub2 	 total
A 	 20 	 30 	 50
B 	 10 	 5 	 15
C 	 25 	 40 	 65
D 	 40 	 10 	 50

Functions

Functions are a small chunk of code which you may call multiple times within your script. They are particularly useful if you have certain tasks which need to be performed several times.

Bash function:

greet() {
    echo Hello $1
}

Invoking the function:

greet A
greet B

Output:

Hello A
Hello B

Previous Next

Problem	Score	Time
Swap Forward And Backward Slash New	200	8:51
Sort by Frequency New	250	71:10
Valid Email Address New	350	34:54
Convert Integer To Roman Number New	400	63:43

Level 2

Regex

Regex metacharacters

Regex Metaclasses:

The following regex will match all words end with ing:

The following regex will match all words which contain quu or quuu and such expression:

Match all words starting with non-vowel and end with a vowel (Usage with grep).

Sed/Awk

Sed

Replace consecutive multiple a with single a

Awk

Calculate total of the students

Functions

Bash function:

Invoking the function:

Output: