minte9
LearnRemember / REGEX



Parentheses

Parentheses can remember text matched by the subexpression they enclose. 
 
#!/bin/sh

: "Backreference
Parentheses can remember text matched by subexpression 
    ([a-z]) \1 
You can have more than one set of parentheses
Use \1, \2, etc to refer to first, second / -o only-matches

Other uses of parantheses:
To limit the scope of alternation (a|b)
To group multiple characters to apply quantifiers (abc)? 
"

A='
the the theory
the theory
an an answer
an answer
cat cat category
cat category
'

echo $A | grep '\<the +the\>'     -o -E | tee result.txt
echo $A | grep '\<(an) +\1\>'     -o -E | tee result.txt -a
echo $A | grep '\<([a-z]+) \1\>'  -o -E | tee result.txt -a
echo $A | grep '\<([a-z]+) \1\>'  -E --colour

 
the the
an an
the the
an an
cat cat






Questions and answers




Can you use parenthesis to group for backreference?

  • a) yes
  • b) no

In regex parenthesis are NOT used to:

  • a) limit the scope of alternation
  • b) negate character class

Multiple backrefenences, lke \1, \2, etc are allowed.

  • a) false
  • b) true

Which regex is used to replace doubled word?

  • a) \b(word) +\1\b
  • b) \b([a-zA-Z]+) +\1\b

Which metasequence is used for backreference? [a,3]

  • a) \1
  • b) $1
  • c) {1}


References