Tcl Tutorial Lesson 21

More Quoting Hell - Regular Expressions 102

regexp ?switches? exp string ?matchVar? ?subMatch1 ... subMatchN?
Searches string for the regular expression exp. If a parameter matchVar is given, then the substring that matches the regular expression is copied to matchVar. If subMatchN variables exist, then the parenthetical parts of the matching string are copied to the subMatch variables, working from left to right.
regsub ?switches? exp string subSpec varName
Searches string for substrings that match the regular expression exp and replaces them with subSpec. The resulting string is copied into varName.

The regular expression (exp) in the two regular expression parsing commands is evaluated by the Tcl parser during the Tcl substitution phase. This can provide a great deal of power, and also requires a great deal of care.

These examples show some of the trickier aspects of regular expression evaluation. The fields in each example are discussed in painful detail in the most verbose level.

The points to remember as you read the examples are:

  • A left square bracket ([) has meaning to the substitution phase, and to the regular expression parser.
  • A set of parentheses, a plus sign, and a star have meaning to the regular expression parser, but not the Tcl substitution phase.
  • A backslash sequence (\n, \t, etc) has meaning to the Tcl substitution phase, but not to the regular expression parser.
  • A backslash escaped character (\[) has no special meaning to either the Tcl substitution phase or the regular expression parser.

The phase at which a character has meaning affects how many escapes are necessary to match the character you wish to match. An escape can be either enclosing the phrase in braces, or placing a backslash before the escaped character. To pass a left bracket to the regular expression parser to evaluate as a range of characters takes 1 escape. To have the regular expression parser match a literal left bracket takes 2 escapes (one to escape the bracket in the Tcl substitution phase, and one to escape the bracket inthe regular expression parsing.). If you have the string placed withinquotes, then a backslash that you wish passed to the regular expression parser must also be escaped with a backslash. Note: You can copy the code and run it in tclsh or wish to see the effects.


Example
  #
  # Examine an overview of UNIX/Linux disks
  #
  set list1 [list \
  {/dev/wd0a        17086    10958     5272    68%    /}\
  {/dev/wd0f       179824   127798    48428    73%    /news}\
  {/dev/wd0h      1249244   967818   218962    82%    /usr}\
  {/dev/wd0g        98190    32836    60444    35%    /var}]

  foreach line $list1 {
      regexp {[^ ]* *([0-9]+)[^/]*(/[a-z]*)} $line match size mnt
      puts "$mnt is $size blocks"
  }
  #
  # Extracting a hexadecimal value ...
  #
  set line {Interrupt Vector? [32(0x20)]}
  regexp "\[^\t]+\t\\\[\[0-9]+\\(0x(\[0-9a-fA-F]+)\\)]" \
      $line match hexval
  puts "Hex Default is: 0x$hexval"

  #
  # Matching the special characters as if they were ordinary
  #
  set str2 "abc^def"
  regexp "\[^a-f]*def" $str2 match
  puts "using \[^a-f] the match is: $match"
  regexp "\[a-f^]*def" $str2 match
  puts "using \[a-f^] the match is: $match"
  regsub {\^} $str2 " is followed by: " str3
  puts "$str2 with the ^ substituted is: \"$str3\""
  regsub "(\[a-f]+)\\^(\[a-f]+)" $str2 "\\2 follows \\1" str3
  puts "$str2 is converted to \"$str3\""

  Resulting output
/ is 17086 blocks
/news is 179824 blocks
/usr is 1249244 blocks
/var is 98190 blocks
Hex Default is: 0x20
using [^a-f] the match is: ^def
using [a-f^] the match is: abc^def
abc^def with the ^ substituted is: "abc is followed by: def"
abc^def is converted to "def follows abc"