String Handling

Ruby hands the method a reference to the caller's String object. strip! edits that object in place, so the caller sees it; name = name.strip builds a new string and rebinds only the method's local parameter.

=~ returns the index where the match starts or nil, match returns a MatchData or nil, and match? returns true or false. =~ and match set $~, $1 and the other match variables; match? sets none of them.

String#encode returns a new string whose bytes are converted so the same characters exist in the target encoding. String#force_encoding keeps the bytes and only relabels the receiver. Relabel when the label is wrong; encode when bytes must change.

Plain <<EOS needs its terminator at column 0 and keeps text as written; <<-EOS lets the terminator be indented; <<~EOS also removes the smallest common indentation from the body. Quoting the identifier as 'EOS' turns off interpolation and escapes.

String#<< appends in place and returns the same object, so every variable holding it sees the change. + returns a new string, and s += x is shorthand for s = s + x, which rebinds only s.

The comment makes every plain string literal in that one file frozen and deduplicated, so mutating one raises FrozenError. Interpolated literals, strings returned by methods, +"" buffers and strings from other files stay mutable.

Ruby's ^ and $ match at the start and end of every line, so a value whose first line is a valid plate passes. \A and \z match only the whole string's start and end, so /\A[A-Z]{2}-\d{3}\z/ rejects extra lines.

String#length (alias size) counts characters as the string's encoding defines them, String#bytesize counts raw bytes, and String#grapheme_clusters splits text into user-perceived characters. Outside ASCII the three numbers can all differ.

chomp removes one trailing line ending (\n, \r or \r\n) and nothing else; chop removes the last character whatever it is; strip removes leading and trailing ASCII whitespace and NUL. All three return new strings.

sub replaces the first match and gsub every match, each returning a new string. A replacement string may use back-references such as \1 or \k<name>, a Hash maps matched text to replacements, and a block computes each replacement.

ASCII-8BIT, aliased BINARY, labels a string as raw bytes with no characters above 0x7F. String#b returns a copy of the same bytes labelled ASCII-8BIT. Joining a binary string holding high bytes with non-ASCII UTF-8 raises Encoding::CompatibilityError.

UndefinedConversionError means a valid source character has no counterpart in the target, such as € into ISO-8859-1. InvalidByteSequenceError means the source bytes are broken for their own encoding. undef: :replace and invalid: :replace substitute a replacement string instead.

format and sprintf are the same Kernel method, and fmt % args calls it with the arguments; %-18s left-justifies in 18 characters and %7.2f prints two decimals. Too few arguments raise ArgumentError, a missing named key raises KeyError.

split with no argument or with " " splits on runs of whitespace and ignores leading whitespace. Any other separator splits on each occurrence, keeping empty fields between separators but dropping trailing ones unless a negative limit is passed.

Name each group with (?<region>…) and (?<number>…), call match, and read md[:region] and md[:number] from the MatchData, after checking it is not nil. named_captures returns all of them as a Hash.

The Latin-1 bytes were labelled with the default external encoding, UTF-8, so the string is invalid and split or any Regexp match raises. Confirm with encoding, valid_encoding? and bytes, then transcode at the boundary with encode("UTF-8", "ISO-8859-1").

A chilled string is a literal from a file with no frozen_string_literal comment. It reports frozen? as false and mutates normally; the first mutation emits a deprecation warning, which is invisible unless deprecation warnings are enabled.

+str returns str itself when it is already mutable without warning, otherwise an unfrozen copy. -str returns a frozen, deduplicated string equal to str, so equal strings share one object. Use + for buffers, - for repeated values.

Regexp.timeout= sets a process-wide limit in seconds, Regexp.new(src, timeout:) sets one per pattern, and an overrun raises Regexp::TimeoutError. Regexp.linear_time? reports whether this interpreter can match a pattern in linear time, for checking your own patterns.

ljust, rjust, center and format widths count characters, not terminal columns. A decomposed accent adds an invisible character and a CJK character fills two columns, so padding by character count leaves rows misaligned.