perl - Substring replacement using regex -

i having terrible time learning perl regular expressions. trying :

replace occurrences of single # @ beginning of line with: #####.
replace occurrences of full line of # characters (ignoring leading or trailing spaces) with
# ---------- #.

i know s/# that's know , can find. suggestions.

the beginning of line matched ^. therefore, line starting # matched by

/^#/

if want # single, i.e. not followed #, must add negative character class:

/^#[^#]/

we not want replace character following #, replace non matching group (called negative look-ahead):

/^#(?!#)/

to add replacement, change

s/^#(?!#)/#####/

the full line can matched following regular expression:

/^#+$/

plus means "once or more", ^ , $ have been explained. have ignore leading , trailing spaces (* means "zero or more"):

/^ *#+ *$/

we not want spaces replaced, have keep them. parentheses create "capture groups" numbered 1:

s/^( *)#+( *)$/$1# ---------- #$2/

Brande