The Match-beginning-of-line Operator (`^')
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  This operator can match the empty string either at the beginning of
the string or after a newline character.  Thus, it is said to "anchor"
the pattern to the beginning of a line.

  In the cases following, `^' represents this operator.  (Otherwise,
`^' is ordinary.)

   * It (the `^') is first in the pattern, as in `^foo'.

   * The syntax bit `RE_CONTEXT_INDEP_ANCHORS' is set, and it is outside
     a bracket expression.

   * It follows an open-group or alternation operator, as in `a\(^b\)'
     and `a\|^b'.  See Grouping Operators, and *Note Alternation
     Operator::.

  These rules imply that some valid patterns containing `^' cannot be
matched; for example, `foo^bar' if `RE_CONTEXT_INDEP_ANCHORS' is set.

  If the `not_bol' field is set in the pattern buffer (*note GNU
Pattern Buffers::.), then `^' fails to match at the beginning of the
string.  See POSIX Matching, for when you might find this useful.

  If the `newline_anchor' field is set in the pattern buffer, then `^'
fails to match after a newline.  This is useful when you do not regard
the string to be matched as broken into lines.