Look ahead and Look behind in perl

Thursday, June 05, 2014 pattern match , perl 2 Comments

With the look-ahead and look-behind constructs ,you can "roll your own" zero-width assertions to fit your needs. You can look forward or backward in the string being processed, and you can require that a pattern match succeed (positive assertion) or fail (negative assertion) there.
Every extended pattern is written as a parenthetical group with a question mark as the first character. The notation for the look-arounds is fairly mnemonic, but there are some other, experimental patterns that are similar, so it is important to get all the characters in the right order.

(?=pattern)

is a positive look-ahead assertion

(?!pattern)

is a negative look-ahead assertion

(?<=pattern)

is a positive look-behind assertion

(?<!pattern)

EXAMPLES
Look-Ahead:

echo $mytmp2
uvw_abc uvw_def uvw_acb

Positive:

echo $mytmp2 | perl -pe 's/uvw_(?=(abc|def))/xyz_/g'
xyz_abc xyz_def uvw_acb

Negative:

echo $mytmp2 | perl -pe 's/uvw_(?!(abc|def))/xyz_/g'
uvw_abc uvw_def xyz_acb

Look-Behind:

echo $mytmp
abc_uvw def_uvw acb_uvw

Positive:

echo $mytmp | perl -pe 's/(?<=(abc|def))_uvw/_xyz/g'
abc_xyz def_xyz acb_uvw

Negative:

echo $mytmp | perl -pe 's/(?<!(abc|def))_uvw/_xyz/g'
abc_uvw def_uvw acb_xyz